He claims that there is a contradiction in the Quran between the verse“There is not one of you but will come to it(Hell)”[Maryam 19:71] and other texts which state that forgiveness and mercy are for those who are killed fighting for the sake of Allah
Question

A Christian put this question to me, and I would like an answer to it so that I can send it to him: in Soorat Maryam it says (interpretation of the meaning): “There is not one of you but will come to it (Hell)” [Maryam 19:71], in the sense that all of mankind will enter Hell for some time, including the Muslims, without exception. Then we find that in Soorat Aal ‘Imraan it indicates that whoever is killed fighting in jihad is not subject to this ruling: “And if you are killed or die in the Way of Allah, forgiveness and mercy from Allah are far better than all that they amass (of worldly wealth, etc.)” [Aal ‘Imraan 3:157]. What is the sound view concerning these two verses? How can I respond to him regarding all these claims? I hope that you can provide me with a detailed answer.
Praise be to Allah.There is no contradiction or conflict in the verses that he has mentioned, or in any other verses of the Quran, praise be to Allah. The words of Allah are far above such a thing. Rather this is applicable to the words of human beings, because of their shortcomings, ignorance and failings. As for the words of the All-Knowing, All Aware, they are far above all faults and shortcomings. Rather the fault is in the understanding of the reader and the mind of the one who says that. 
The response to that is that the verse in Aal ‘Imraan does not contradict the verse in Maryam, and it does not mean at all that the coming to Hell that is mentioned in Maryam is not real, because it is certain truth, and will definitely happen. One of the things which indicate that is the report narrated from Abu Hurayrah (may Allah be pleased with him), from the Prophet (blessings and peace of Allah be upon him), who said: “No Muslim, three of whose children die, will come to the Fire, except for the fulfilment of the oath.” 
Narrated by al-Bukhaari (1193) and Muslim (2632) 
Imam al-Bukhaari (may Allah have mercy on him) explained the words “fulfilment of the oath” by quoting, at the end of the hadith, the words: “There is not one of you but will come to it (Hell)” [Maryam 19:71]. 
An-Nawawi (may Allah have mercy on him) said: The scholars said: With regard to the words “fulfilment of the oath”, it was said in the explanation of the hadith that what is referred to is the verse in which Allah, may He be exalted, says (interpretation of the meaning): “There is not one of you but will come to it (Hell)” [Maryam 19:71]. This is the view of Abu ‘Ubayd and the majority of scholars. What it means is: By Allah, there is not one of you but will come to it. And it was said that what is referred to is the verse in which Allah, may He be exalted, says (interpretation of the meaning): “So by your Lord, surely, We shall gather them together, and (also) the Shayatin (devils) (with them)” [Maryam 19:68].
End quote from Sharh Muslim (16/180). 
Once it is clear to us that there is no contradiction between the verses, and that the coming to Hell is something that is applicable to all of mankind, even if they are Muslims, then it remains for us to find out what is meant by the “coming to” mentioned in the verse in Soorat Maryam. So we say that the strongest views on the meaning of the “coming to” mentioned in the verse in Soorat Maryam are two views, as follows: 
The first view is that what is meant is entering, and that the righteous will be safe from its heat and flames, and the Lord of the Worlds will leave in it the wrongdoers among the disbelievers and those among the Muslims who deserve punishment in it. As for the disbelievers, their punishment will last for ever, whereas the Muslims will be punished for a limited time. The evidence for that is what is mentioned after this verse, where Allah, may He be exalted, says (interpretation of the meaning): “Then We shall save those who used to fear Allah and were dutiful to Him. And We shall leave the Zalimoon (polytheists and wrongdoers, etc.) therein (humbled) to their knees (in Hell)” [Maryam 19:72]. This is the view of Ibn ‘Abbaas (may Allah be pleased with him), and was regarded as more likely to be correct by Shaykh ash-Shinqeeti. Anyone wishing to know more may read his words in his tafseer, Adwa’ al-Bayaan (3/478-481). 
The second view is that it is a “coming to” that is only for the Muslims who affirmed the Oneness of Allah, whether they were people who did acts of obedience and worship, or they were people who did acts of disobedience and committed sin. This “coming to” does not mean entering the Fire; rather it means passing above it, and that will be by crossing over the Siraat, which is a bridge that will be set up over Hell, that only the Muslims will cross over, and some of them will be saved and some of them will fall into the Fire. As for the disbelievers, they will not pass over the Siraat, because they will enter Hell directly before that in humiliation. This is the view of Ibn Mas‘ood (may Allah be pleased with him) and was regarded as more likely to be correct by many of the scholars. 
Ibn Rajab al-Hanbali (may Allah have mercy on him) said:
It should be understood that the people will be divided into (two groups): believers who worshipped Allah alone and did not associate anything with Him, and mushrikeen who worshipped others besides Allah. As for the mushrikeen, they will not pass over the Siraat; rather they will fall into Hell before the Siraat is set up.
End quote from at-Takhweef min an-Naar (p. 233) 
So the coming to Hell – according to this second view – when mentioned with regard to the Muslims, as in the verse in Soorat Maryam where Allah, may He be exalted, says, “There is not one of you…”, refers to passing over Hell. As for the “coming to” mentioned with regard to the disbelievers, it means entering it. 
So the “coming to” [wurood] is of two types: coming to and entering into a place, and coming to it whilst being close to it, reaching it and looking into it, without entering it. Both meanings are mentioned in the Book of Allah, may He be exalted. As for the former, it appears with reference to the disbelievers and sinners who are subject to the warning. Concerning this, Allah, may He exalted, says (interpretation of the meaning):
“Certainly! You (disbelievers) and that which you are worshipping now besides Allah, are (but) fuel for Hell! (Surely), you will enter it [laha waaridoon].
Had these (idols, etc.) been aliha (gods), they would not have entered there [ma waraduha] (Hell), and all of them will abide therein”
[al-Anbiya’ 21:98, 99]
“He will go ahead of his people on the Day of Resurrection, and will lead them [fa awradahum] into the Fire, and evil indeed is the place to which they are led”
[Hood 11:98].
 Ibn Rajab (may Allah be pleased with him) said:
If a person is put together in the punishment with the one who was the cause of his punishment, that will exacerbate his pain and regret.
End quote from at-Takhweef min an-Naar (p. 99) 
With regard to “coming to” [wurood] in the second sense, examples of that include the verse in which Allah, may He be exalted, says (interpretation of the meaning):
“And when he arrived [warada] at the water of Madyan (Midian) he found there a group of men watering (their flocks)”
[al-Qasas 28:23].           
 As the verse in Soorat Maryam is applicable to all the Muslims, in that case the “coming to” mentioned there is to be understood according to the second meaning. As for the first meaning, it is not applicable to all Muslims; rather it is applicable to those of them who deserve to enter Hell, and is also applicable to the disbelievers. 
Shaykh ‘Umar Sulaymaan al-Ashqar (may Allah have mercy on him) said: Some of the scholars are of the view that what is meant by coming to the Fire, as mentioned in the verse in which Allah, may He be exalted, says (interpretation of the meaning): “There is not one of you but will come to it (Hell); this is, with your Lord, a Decree which must be accomplished” [Maryam 19:71], is entering the Fire. This is the view of Ibn ‘Abbaas (may Allah be pleased with him), who quoted as evidence for that the words of Allah, may He be exalted, concerning Pharaoh (interpretation of the meaning): “He will go ahead of his people on the Day of Resurrection, and will lead them [fa awradahum] into the Fire, and evil indeed is the place to which they are led” [Hood 11:98]; and the verses in which Allah says (interpretation of the meaning): “And We shall drive the Mujrimoon (polytheists, sinners, criminals, disbelievers in the Oneness of Allah, etc.) to Hell, in a thirsty state (like a thirsty herd driven down to water) [wirdan]” [Maryam 19:86] and “Had these (idols, etc.) been aliha (gods), they would not have entered there (Hell) [ma waraduha]” [al-Anbiya’ 21:99]. And Muslim al-A‘war narrated from Mujaahid concerning the words “There is not one of you but will come to it (Hell)” [Maryam 19:71] that he said: He will enter it. 
Other scholars were of the view that what is meant by “coming to” here is passing over the siraat. The commentator on at-Tahhaawiyyah said: The mufassireen differed as to what is meant by coming to [wurood] in the verse, “There is not one of you but will come to it (Hell)” [Maryam 19:71]. The strongest view and the one that is most likely to be correct is that what is meant is passing over the siraat. Allah, may He be exalted, says (interpretation of the meaning): “Then We shall save those who use to fear Allah and were dutiful to Him. And We shall leave the Zalimoon (polytheists and wrongdoers, etc.) therein (humbled) to their knees (in Hell)” [Maryam 19:72]. 
In as-Saheeh it is narrated that the Prophet (blessings and peace of Allah be upon him) said: By the One in Whose hand is my soul, no one will enter Hell who swore allegiance beneath the tree.” Hafsah said: I said: O Messenger of Allah, didn’t Allah say, “There is not one of you but will come to it (Hell)” [Maryam 19:71]? He said: Did you not hear that He said: “Then We shall save those who use to fear Allah and were dutiful to Him. And We shall leave the Zalimoon (polytheists and wrongdoers, etc.) therein (humbled) to their knees (in Hell)” [Maryam 19:72]? 
The Prophet (blessings and peace of Allah be upon him) indicated that coming to the Fire does not necessarily mean entering it, and that being saved from its evil does not necessarily mean after entering it. Rather it means that the reasons for entering it are there, but they will be saved from it. If a person was pursued by his enemies, who wanted to kill him, but they did not manage to do so, it may be said that Allah saved him from them. Hence Allah, may He be exalted, says (interpretation of the meaning): “And when Our Commandment came, We saved Hood” [Hood 11:58]; “So when Our Commandment came, We saved Saalih” [Hood 11:66]’ “And when Our Commandment came, We saved Shu‘ayb” [Hood 11:94]. The punishment did not befall them, but it befell others. Were it not for the fact that Allah made the means of salvation available to them only, the same fate would have befallen them as befell others. The same applies to those who come to the Fire and pass over it on the siraat, then Allah will save those who feared Him, and will leave the wrongdoers therein (humbled) to their knees. The Prophet (blessings and peace of Allah be upon him) explained, in the hadith of Jaabir mentioned above, that “coming to” (wurood) here refers to passing over the siraat. End quote. 
In fact, coming to the Fire means two things: the first of which is the coming of the disbelievers, who are the inhabitants of the Fire. In their case coming to means entering it, beyond any doubt, as Allah, may He be exalted, says regarding Pharaoh (interpretation of the meaning): “He will go ahead of his people on the Day of Resurrection, and will lead them [fa awradahum] into the Fire, and evil indeed is the place to which they are led” [Hood 11:98], that is, evil indeed is the place which they will enter.
The second meaning of coming to the Fire is that of those who affirmed the oneness of Allah. That is, their passing over the siraat in the manner mentioned in the hadiths.
End quote from al-Qiyaamah al-Kubra (p. 267, 268) 
Whether we go by the first view or the second – which is more likely to be correct in our opinion – there is no contradiction between the texts of the revelation, praise be to Allah, the Lord of the Worlds. 
And Allah knows best.