A Christian has come across a mention of our Prophet (blessings and peace of Allah be upon him) in their scripture
Question

I am a young Christian man and I have studied the Islamic and Christian religions. I found that the Prophet Muhammad is mentioned in the Gospel, but none of the Muslims want to know the truth and the Christians reject what I say. What should I do?
Praise be to Allah.Dear young man, what good will it do you if all the people are saved and you are doomed? If everyone on earth became Muslim because of learning about what you have found in the Gospel, and they entered Paradise and were saved from Hell, but you did not benefit from that, and you continued to adhere to distorted Christianity until you died following it – God forbid – then, by your Lord, what benefit could you hope for then?
What is in store for you is something else.
Forget about the Muslims and the Christians, and think: what are you waiting for, now that you have seen the truth and you learned about the Prophethood of the Prophet Muhammad (blessings and peace of Allah be upon him) in your own scripture?
Do you not know that the messages of Allah to His slaves, and His guidance to the people of earth, were not completed, and the blessing was not perfected, until Allah sent His Prophet Muhammad (blessings and peace of Allah be upon him) to complete what his fellow Prophets  had brought before him of the call to the Lord of the Worlds?
Allah, may He be exalted, says (interpretation of the meaning):
“This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islam as your religion”
[al-Maa’idah 5:3].
It was narrated from Jaabir ibn ‘Abdullah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “My likeness and that of the Prophets is that of a man who built a house and completed and perfected it, except for the space of one brick. The people started to enter it and admire it, saying: Were it not for the space of this brick.” The Messenger of Allah (blessings and peace of Allah be upon him) said: “I am that brick; I have come to complete (the mission of) the Prophets.” Agreed upon. 
Do you not know that Allah took a covenant from the Prophets and from the believers who followed them, that they would believe in the final Prophet of Allah, Muhammad (blessings and peace of Allah be upon him), if they lived to see his message and news of his Prophethood reached them, and they came to know of his religion and his book?
Allah, may He be exalted, says (interpretation of the meaning):
“And (remember) when Allah took the Covenant of the Prophets, saying: ‘Take whatever I gave you from the Book and Hikmah (understanding of the Laws of Allah, etc.), and afterwards there will come to you a Messenger (Muhammad (blessings and peace of Allah be upon him)) confirming what is with you; you must, then, believe in him and help him.’ Allah said: ‘Do you agree (to it) and will you take up My Covenant (which I conclude with you)?’ They said: ‘We agree.’ He said: "Then bear witness; and I am with you among the witnesses (for this)’”
[Aal ‘Imraan 3:81].
There are many of your fellow Christians who were guided, who hastened to follow the truth that they found in their scripture, and no threat scared them and no temptation deterred them – men and women, Arabs and non-Arabs, leaders and common folk; let them be a beacon on your path.
Let these Christians from Abyssinia be an example for you too, for as soon as they heard true guidance, they realised that it was the truth that confirmed what they had of the Torah and Gospel, so they became Muslim and followed the unlettered Prophet (blessings and peace of Allah be upon him). Allah, may He be exalted, says (interpretation of the meaning):
“Verily, you will find the strongest among men in enmity to the believers (Muslims) the Jews and those who are Al-Mushrikoon, and you will find the nearest in love to the believers (Muslims) those who say: ‘We are Christians.’ That is because amongst them are priests and monks, and they are not proud.
And when they (who call themselves Christians) listen to what has been sent down to the Messenger (Muhammad (blessings and peace of Allah be upon him)), you see their eyes overflowing with tears because of the truth they have recognised. They say: ‘Our Lord! We believe; so write us down among the witnesses.
‘And why should we not believe in Allah and in that which has come to us of the truth (Islamic Monotheism)? And we wish that our Lord will admit us (in Paradise on the Day of Resurrection) along with the righteous people (Prophet Muhammad (blessings and peace of Allah be upon him) and his Companions ).’
So because of what they said, Allah rewarded them Gardens under which rivers flow (in Paradise), they will abide therein forever. Such is the reward of gooddoers”
[al-Maa’idah 5:82-85].
Why should they not be like that, when their king before them had hastened to follow the truth when he saw it? He was the Negus, the king of Abyssinia. Listen to his story, as told by someone who was there at that time. It was narrated that Abu Moosa (may Allah be pleased with him) said:
The Messenger of Allah (blessings and peace of Allah be upon him) instructed us to set out for the land of Abyssinia. News of that reached Quraysh, so they sent word to ‘Amr ibn al-‘Aas and ‘Umaarah ibn al-Waleed, who collected gifts for the Negus. We and they came to the Negus, and they brought a gift, which he accepted, and they prostrated to him. Then ‘Amr ibn al-‘Aas said: Some of our people have turned away from our religion, and they are in your land. The Negus said to them: In my land? He said: Yes. So he sent for us, and Ja‘far said to us: No one of you should speak; I will be your spokesman today. We came to the Negus when he was sitting in his court, with ‘Amr ibn al-‘Aas on his right and ‘Umaarah on his left, and the priests and monks were sitting in two rows. ‘Amr and ‘Umaarah said to him: They will not prostrate to you. When we came to him, the priests and monks who were with him said: Prostrate to the king. Ja‘far said: We only prostrate to Allah. The Negus said to him: Why is that? He said: Allah has sent to us His Messenger; he is the Messenger who was foretold by ‘Eesaa (Jesus), who said that a Messenger would come after him whose name would be Ahmad. He instructed us to worship Allah and not associate anything with Him, and to establish prayer and give zakaah. And he instructed us to do what is right and forbade us to do what is wrong. The people were impressed by his words, and when ‘Amr saw that, he said: May Allah bless the king; they disagree with you concerning ‘Eesa ibn Maryam (Jesus son of Mary). The Negus said to Ja‘far: What does your companion say about the son of Maryam? He said: He says concerning him what Allah says: He is a soul created by Allah and His word whom he brought forth from the Virgin whom no man had touched. The Negus picked up a stick from the ground and held it up, then he said: O priests and monks, what these people say about the son of Maryam does not differ from what you say by as much as the weight of this stick. Welcome to you and the one from whom you have come. I bear witness that he is the Messenger of Allah and that he is the one who was foretold by ‘Eesaa ibn Maryam. Were it not for my position of kingship, I would go to him and carry his sandals. Stay in my land for as long as you want. And he ordered that they be given food and clothing, and he said: Return their gift to these people (Quraysh). 
Narrated by al-Haakim in al-Mustadrak (3208)
Do you not know that if you believed in your former religion, then the truth became clear to you and you believed in the religion of Islam, and followed its Messenger, the last of the Prophets and Messengers, you would have a double reward? Allah, may He be exalted, says (interpretation of the meaning):
“Those to whom We gave the Scripture (i.e. the Taurat (Torah) and the Injeel (Gospel), etc.) before it, - they believe in it (the Quran).
And when it is recited to them, they say: ‘We believe in it. Verily, it is the truth from our Lord. Indeed even before it we have been from those who submit themselves to Allah in Islam as Muslims’ (like Abdullah bin Salam and Salman Al-Farisee, etc.).
These will be given their reward twice over, because they are patient, and repel evil with good, and spend (in charity) out of what We have provided them.
And when they hear AlLaghw (dirty, false, evil vain talk), they withdraw from it and say: ‘To us our deeds, and to you your deeds. Peace be to you. We seek not the ignorant’”
[al-Qasas 28:52-55]. 
It was narrated from Abu Moosa al-Ash‘ari (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “There are three who will be given a double reward: a man from among the people of the Book who believed in his Prophet, then lived to see the Prophet (blessings and peace of Allah be upon him) and followed him and believed in him– he will have a double reward; and a slave who fulfils his duty towards Allah and towards his master – he will have a double reward; and a man who had a slave woman whom he fed and fed her well, and taught her and taught her well, then he set her free and married her – he will have a double reward.”.
Narrated by al-Bukhaari (3011) and Muslim (154).
Finally, the Lord, may He be glorified and exalted, said in the Last Testament, i.e., the Qur’an (interpretation of the meaning):
“O mankind! Verily, there has come to you the Messenger (Muhammad (blessings and peace of Allah be upon him)) with the truth from your Lord, so believe in him, it is better for you. But if you disbelieve, then certainly to Allah belongs all that is in the heavens and the earth. And Allah is Ever All-Knowing, All-Wise.
O people of the Scripture (Jews and Christians)! Do not exceed the limits in your religion, nor say of Allah aught but the truth. The Messiah ‘Eesa (Jesus), son of Maryam (Mary), was (no more than) a Messenger of Allah and His Word, (‘Be!’ - and he was) which He bestowed on Maryam (Mary) and a spirit (Rooh) created by Him; so believe in Allah and His Messengers. Say not: ‘Three (trinity)!’ Cease! (it is) better for you. For Allah is (the only) One Ilah (God), Glory be to Him (Far Exalted is He) above having a son. To Him belongs all that is in the heavens and all that is in the earth. And Allah is All-Sufficient as a Disposer of affairs.
The Messiah will never be proud to reject to be a slave to Allah, nor the angels who are near (to Allah). And whosoever rejects His worship and is proud, then He will gather them all together unto Himself.
So, as for those who believed (in the Oneness of Allah - Islamic Monotheism) and did deeds of righteousness, He will give their (due) rewards, and more out of His Bounty. But as for those who refuse His worship and were proud, He will punish them with a painful torment. And they will not find for themselves besides Allah any protector or helper.
O mankind! Verily, there has come to you a convincing proof (Prophet Muhammad (blessings and peace of Allah be upon him)) from your Lord, and We sent down to you a manifest light (this Quran).
So, as for those who believed in Allah and held fast to Him, He will admit them to His Mercy and Grace (i.e. Paradise), and guide them to Himself by a Straight Path”
[an-Nisa’ 4:170-175].
We ask Allah to guide you to that which is best in this world and the hereafter, and to open your heart to guidance and the religion of truth.
For more information, please see the answer to question number 154022 and other questions to which it refers.
And Allah knows best.