Ruling on studying the books of the People of the Book for the purpose of da‘wah (calling them to Islam), and the ruling on studying comparative religion.
Question

What ruling on studying the books of the People of the Book for the purpose of da‘wah (calling them to Islam), and the ruling on studying comparative religion?
Praise be to Allah.Firstly: 
The Torah and Gospel were originally from Allah, may He be exalted, and we are obliged to believe in them because Allah, may He be exalted, says (interpretation of the meaning):
“Say (O Muslims), ‘We believe in Allah and that which has been sent down to us and that which has been sent down to Ibraaheem (Abraham), Ismaa‘eel (Ishmael), Ishaaq (Isaac), Ya‘qoob (Jacob), and to Al-Asbaat (the twelve sons of Ya‘qoob (Jacob)), and that which has been given to Moosa (Moses) and ‘Eesa (Jesus), and that which has been given to the Prophets from their Lord. We make no distinction between any of them, and to Him we have submitted (in Islam)’” [al-Baqarah 2:136]
“O you who believe! Believe in Allah, and His Messenger (Muhammad (blessings and peace of Allah be upon him)), and the Book (the Quran) which He has sent down to His Messenger, and the Scripture which He sent down to those before (him), and whosoever disbelieves in Allah, His Angels, His Books, His Messengers, and the Last Day, then indeed he has strayed far away”
[an-Nisa’ 4:136]. 
But the Torah and Gospel were subjected to distortion and changes. Allah, may He be exalted, says (interpretation of the meaning):
“Then woe to those who write the Book with their own hands and then say, "This is from Allah," to purchase with it a little price! Woe to them for what their hands have written and woe to them for that they earn thereby”
[al-Baqarah 2:79]. 
Because of this distortion and the mixing of truth with falsehood in these two Scriptures, it is prohibited to study them. 
It was narrated from Jaabir ibn ‘Abdullah (may Allah be pleased with him) that ‘Umar ibn al-Khattaab (may Allah be pleased with him) came to the Prophet (blessings and peace of Allah be upon him) with some written material he had got from one of the people of the Book. He read it to the Prophet (blessings and peace of Allah be upon him), and he got angry and said: “Are you confused (about your religion), O son of al-Khattaab? By the One in Whose hand is my soul, I have brought it (the message of Islam) to you clear and pure. Do not ask them about anything, lest they tell you something true and you disbelieve it, or they tell you something false and you believe it. By the One in Whose hand is my soul, if Moosa were alive, he would have no option but to follow me.”
Narrated by Ahmad (14736); classed as hasan by al-Albaani in Irwa’ al-Ghaleel, 6/34 
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said, after mentioning the isnaads of this hadith: 
These are all the isnads of this hadith; even though it does not reach such a standard that it may be relied upon as evidence, when they are all taken into consideration, this implies that there is a basis for this report.
End quote from Fath al-Baari (13/525) 
Moreover, the truth that we have in the Holy Quran means that there is no need for whatever truth may be in the Torah and Gospel. 
Allah, may He be exalted, says (interpretation of the meaning):
“Is it not sufficient for them that We have sent down to you the Book (the Quran) which is recited to them? Verily, herein is mercy and a reminder (or an admonition) for a people who believe”
[al-‘Ankaboot 29:51]. 
Ibn Taymiyah (may Allah have mercy on him) said: 
Because the Quran is the best of speech, they were prohibited to follow anything else. Allah, may He be exalted, says (interpretation of the meaning):
“Is it not sufficient for them that We have sent down to you the Book (the Quran) which is recited to them? Verily, herein is mercy and a reminder (or an admonition) for a people who believe”
[al-‘Ankaboot 29:51]. 
An-Nasaa’i and others narrated from the Prophet (blessings and peace of Allah be upon him) that he saw a page of the Torah in the hand of ‘Umar ibn al-Khattaab (may Allah be pleased with him). He said: “If Moosa were alive, and you were to follow him and leave me, you would go astray.” According to another report: “… he would have no option but to follow me.” According to another version: The face of the Prophet (blessings and peace of Allah be upon him) changed when ‘Umar showed that to him, and one of the Ansaar said to him: O son of al-Khattaab, do you not see the face of the Messenger of Allah (blessings and peace of Allah be upon him)? ‘Umar said: I am content with Allah as my Lord, Islam is my religion, and Muhammad as my Prophet. 
Hence the Sahaabah forbade the study of any book other than the Quran.
End quote from Majmoo‘ al-Fatawa (17/41-42) 
Hence Ibn ‘Abbaas (may Allah be pleased with him) used to say: 
How can you ask the people of the Book about anything, when your Book that was revealed to the Messenger of Allah (blessings and peace of Allah be upon him) is more recent, and you read it pure and not tampered with? And He has told you that the People of the Book altered the Book of Allah and changed it, and they wrote the Book with their own hands, and said that it is from Allah, to purchase with it a little price. Does not the knowledge that has come to you forbid you to ask them about anything? No, by Allah, we have not seen any man among them ask you about that which was revealed to you.
Narrated by al-Bukhari (7363). 
Moreover, focusing on the Torah and Gospel is a distraction from that which will benefit the Muslim with regard to his Hereafter. 
Al-Qurtubi (may Allah have mercy on him) said: 
If the one who reads it – namely the Quran – will have a tenfold reward or more for each letter, according to what we mentioned in the introduction to this book, then turning away from it and towards other Books is misguidance and loss, and it is a bad deal and waste of time.
End quote from al-Jaami‘ li Ahkaam al-Quran (16/37 8) 
Secondly: 
Based on the above, the scholars (may Allah have mercy on them) are of the view that those who study these books – the books of the Jews and Christians – may be divided into two groups: 
-1-
The first group is ordinary people, those who have no knowledge and those who are weak in faith. They are not allowed to study these books so that they will not be confused by that which has been introduced into them of falsehood, and so that they will not be distracted by something that is of no benefit. 
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 
With regard to this matter, it is important to note that in the case of those who are not well-versed in knowledge and are lacking in faith, it is not permissible for them to read any of those books.
End quote from Fath al-Baari (13/525) 
With regard to this group – ordinary people and those who come under the same heading, or those who seek to benefit in spiritual terms from those books – these are the ones who the scholars stated should not be allowed to read the Torah and Gospel. 
It says in Mataalib Ooli an-Nuha (1/607): 
It is not permissible to study the books of the people of the Book, based on textual evidence, because the Prophet (blessings and peace of Allah be upon him) got angry when he saw ‘Umar (may Allah be pleased with him) carrying a page of the Torah. … And they should not read books that contain both truth and falsehood, or narrate what they learn from them, because that may be detrimental to belief. End quote. 
An-Nawawi (may Allah have mercy on him) said: 
The books of the Torah and Gospel are among the things that it is prohibited to seek benefit therein, because they changed and altered them.
End quote from Rawdat at-Taalibeen (10/259) 
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) said:
Every Muslim is obliged to believe in them and that they (originally) came from Allah: the Torah, the Gospel and the Psalms (Zaboor). So he believes that Allah sent down the Books to the Prophets, and He sent down to them Scriptures containing commands and prohibitions, exhortation and reminders, stories of some events in the past, descriptions of Paradise and Hell, and so on. But he should not read them, because distortions, alterations and changes have been introduced into them. So he should not keep copies of the Torah, Gospel or Psalms, or read them, because doing that is risky, as it may lead to him disbelieving something that is true or believing something that is false. That is because these books have been distorted and changed, and those Jews, Christians and others have introduced alterations and distortions into them, changing the order of things in some cases. Allah has given us a great Book which means that we have no need of them, namely the Holy Quran.
End quote from Fatawa Noor ‘ala ad-Darb (1/9) 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
is it permissible for a Muslim to keep a copy of the Gospel in order to find out what Allah said to His slave and Messenger ‘Eesa (peace be upon him)? 
He replied: 
It is not permissible to keep anything of the Scriptures that came before the Quran, be it the Gospel, Torah or anything else, for two reasons:
1. because everything in them that is beneficial Allah, may He be glorified and exalted, has explained it in the Holy Quran;
2. there is no need for any of these books because we have the Quran. Allah, may He be exalted, says (interpretation of the meaning): “It is He Who has sent down the Book (the Quran) to you (Muhammad ) with truth, confirming what came before it” [Aal ‘Imraan 3:3] and “And We have sent down to you (O Muhammad (blessings and peace of Allah be upon him)) the Book (this Quran) in truth, confirming the Scripture that came before it and Muhayminan (trustworthy in highness and a witness) over it (old Scriptures). So judge between them by what Allah has revealed” [al-Maa’idah 5:48]. Whatever of good there is in the previous scriptures is also to be found in the Quran. 
With regard to the questioner saying that he wants to find out what Allah said to His slave and Messenger ‘Eesa, we can find what is most beneficial of that in what Allah has told us in the Quran, so there is no need to look elsewhere. Moreover, the Gospel as it exists at present has been distorted. The evidence for that is that there are four Gospels which differ from one another; there is not one single Gospel, therefore it cannot be relied upon.
End quote from Majmoo‘ Fatawa wa Rasaa’il Ibn ‘Uthaymeen (1/32-33) 
-2-
The second category of people is those who are well versed in knowledge, who use these books to debate with the Jews and Christians, and to establish proof against them. 
In this case the fear of confusion is less, because the one who is well versed in knowledge has the ability to recognise the falsehood that has been introduced into these books and to be cautious of it, and to refute it and warn others against it. Moreover, by refuting the people of the Book and their falsehood, he is serving an important shar‘i interest that must be served. 
Hence many scholars read these books for the purpose of debating with the Jews and Christians. Among the most famous of those who refuted the Jews and Christians by quoting their own books to them were: Ibn Taymiyah, in his book al-Jawaab as-Saheeh liman baddala Deen al-Maseeh (The Correct Response to those who altered the Religion of the Messiah); Ibn al-Qayyim in his book  Hidaayat al-Hayaara fi Ajwibat al-Yahood wa’n-Nasaara (Guiding to the Confused in Refuting the Jews and Christians); Ibn Hazm in his book al-Fasl fi’l-Milal wa’l-Ahwa’ wa’n-Nahl (Discussion of the Views of Different Religions, Groups and Sects); al-Qurtubi in his book al-I‘laam bima fi Deen an-Nasaara min al-Fasaad wa’l-Awhaam (Highlighting what the Religion of the Christians contains of Corruption and Myths); and many others. 
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 
… unlike the one who is well versed in knowledge. For him it is permissible to read in the Torah and Gospel, especially when he needs to refute an opponent. This is indicated by the fact that the imams (leading scholars), both in the past and more recently, quoted from the Torah to prove to the Jews that they should believe in Muhammad (blessings and peace of Allah be upon him) on the basis of what they found in their book. If they did not believe that it was permissible to read it, they would not have done that and persisted in doing so, generation after generation.
End quote from Fath al-Baari (13/525-526) 
Ibn Taymiyah (may Allah have mercy on him) said: 
If it so happens that there are some of the people of the Book who convert to Islam, and they have knowledge of what they have in their language and can translate it to us in Arabic, that may be useful when debating with the Jews and Christians or talking to them, as ‘Abdullah ibn Salaam, Salmaan al-Faarisi, Ka‘b al-Ahbaar and others did; they transmitted what they had of knowledge. In that case we may quote what they have that is in harmony with what the Messenger brought, and that will be proof against them in some ways and against others in other ways, as we have explained elsewhere.
End quote from Majmoo‘ al-Fatawa (4/109-110) 
In Mataalib Ooli an-Nuha (1/607-608), which is a Hanbali book, it says: 
The idea of it being permissible to read in the books of the innovators may be applicable for one who is well versed in the Quran and Sunnah, if he is also very steadfast, strong in faith, smart and alert, very intelligent, and able to derive evidence in order to refute them, discover their secrets and expose them, so that ignorant people will not be taken in by their deceiving arguments and thus become confused. This was done by leading scholars among the best of the Muslims, and they presented strong arguments against the innovators that the latter could not find an answer to. Similarly, they looked in the Torah and found in it mention of our Prophet in many locations; the permissibility of looking in their Book is also applicable in that case. End quote. 
The Standing Committee for Academic Research and Issuing Fatwas was asked: what is the ruling on reading the Gospel? 
They replied:
The earlier divinely revealed Books have been subjected to a great deal of distortion, additions and subtractions, as Allah has told us. Hence it is not permissible for the Muslim to read them and study them, unless he is one of those who are well versed in knowledge and wants to highlight the distortions and contradictions in those books.
End quote from Fatawa al-Lajnah ad-Daa’imah – vol. 1 (3/311) 
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) said:
It is permissible for the well versed scholar to read them in order to refute the opponents of Islam among the Jews and Christians, as the Prophet (blessings and peace of Allah be upon him) called for the Torah when the Jews denied the ruling of stoning, so that he could look at it, and after that they admitted it. 
The point is that the scholars who have knowledge of Islam may need to look at the Torah, Gospel or Psalms for an Islamic purpose, such as refuting the enemies of Allah, or highlighting the virtue of the Quran and what it contains of truth and guidance. As for the common folk and others like them, they should not do any such thing. Rather if they have anything of the Torah, Gospel or Psalms, they must bury it in a clean place or burn it, so that no one will be led astray by it.
End quote from Fatawa Noor ‘ala ad-Darb (1/10) 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
As for the seeker of knowledge who has sufficient knowledge that enables him to distinguish truth from falsehood, there is nothing wrong with him learning about them – namely the Gospels – in order to refute what they contain of falsehood and establish proof against their followers.
End quote from Majmoo‘ Fatawa wa Rasaa’il Ibn ‘Uthaymeen (1/33) 
To sum up, for the daa‘iyahs who call people to Allah, may He be exalted, to use the texts of the Torah and Gospel in order to refute the Jews and Christians and highlight their misguidance, this is something that is prescribed and is not an innovation. Rather it is a matter for which there is a basis in the Quran and Sunnah. 
Ibn Taymiyah (may Allah have mercy on him) said: 
If someone among them is disputing and wants to quote something to undermine the Quran on a textual or rational basis, such as quoting something from their books about the Prophets that is contrary to what was brought by Muhammad (blessings and peace of Allah be upon him), or is contrary to what Allah said in their Books, such as when they told the Prophet (blessings and peace of Allah be upon him) that Allah had commanded them to blacken the face of the adulterer, and not stone him, the Prophet (blessings and peace of Allah be upon him) and the believers were able to ask for the Torah and for a trustworthy translator who could read it and translate it into Arabic, such as ‘Abdullah ibn Salaam and others like him. When he said to their rabbi: “Lift up your hand and expose the verse of stoning,” the verse became visible, and the Prophet (blessings and peace of Allah be upon him) stoned the two Jewish adulterers, after having established proof against them from their own book that was in accordance with what Allah had revealed to him about stoning. 
Likewise, it is possible to read from a translation of it in order to learn what they have, via a trustworthy Muslim translator or those among us who know their language, as in the case of Zayd ibn Thaabit and others like him. … Hence Allah, may He be exalted, said (interpretation of the meaning): “All food was lawful to the Children of Israel, except what Israel made unlawful for himself before the Tawraat (Torah) was revealed. Say (O Muhammad (blessings and peace of Allah be upon him)): "Bring here the Tawraat (Torah) and recite it, if you are truthful” [Aal ‘Imraan 3:93]. 
Allah has instructed us to ask them to bring the Torah and recite it, if they are being truthful in quoting something that is contrary to that. For they used to “distort the Book with their tongues (as they read), so that you may think it is from the Book, but it is not from the Book” [Aal ‘Imraan 3:78], and “write the Book with their own hands and then say, ‘This is from Allah’” [al-Baqarah 2:79]. So they used to lie in what they said and what they wrote; hence a translation can only be accepted from one who is trustworthy. 
If one of them quotes a report from the previous Messengers as evidence for something that is contrary to the Quran, such as those who say that Moosa said, “Adhere to observance of the Sabbath so long as heaven and earth endure,” we can say to them: “In which book is this? Bring it (and show us).” For we know that this is not in their Books; rather it is nothing but a fabrication. 
End quote from Majmoo‘ al-Fatawa (4/110-112) 
And Allah knows best.