I was wondering if anyone could help me out with a question I was posed by a non-Islamic teacher of mine. His question was: "You, Muslims claim that the Quran is perfect and without any contradictions, right? Then why in Surah 7 verse 157 when God speaks to Moses He says: (7:157 those who follow the messenger, the Prophet who can neither read nor write, whom they will find described in the Torah and the Gospel (which are) with them. He will enjoin on them that which is right and forbid them that which is wrong. He will make lawful for them all good things and prohibit for them only the foul; and he will relieve them of their burden and the fetters that they used to wear. Then those who believe in him, and honor him and help him, and follow the light which is sent down with him: they are the successful). Picktall The Gospel didn't exist at that time. So why is there said that: (..."Gospel (which are) with them"

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The verse which you mentioned in the question and the one which comes before it is the Saying of Allaah (which means): {[Allaah] said, “My punishment, I afflict with it whom I will, but My Mercy encompasses all things.” So I will decree it [especially] for those who fear Me and give Zakat and those who believe in Our verses. Those who follow the Messenger, the unlettered prophet, whom they find written [i.e. mentioned] in what they have of the Torah and the Gospel, who enjoins upon them what is right and forbids them what is wrong and makes lawful for them the good things and prohibits for them the evil and relieves them of their burden and the shackles which were upon them. So they who have believed in him, honoured him, supported him and followed the light which was sent down with him, it is those who will be the successful.}[Quran 7:156-157]. The meaning of the Saying of Allaah (which means): {So I will decree it [especially] for those who fear Me and give Zakat and those who believe in Our verses.} refers to the children of Israel who lived at the time of the Prophet and after this time, and who believed in him and it does not refer to the children of Israel at the time of Moosaa (Moses) . So the meaning of the verse is that Allaah informed Moosaa that He will decree His Mercy on the Jews and Christians who will believe in the Prophet when he is sent as a Prophet or after his period, and they can find his description in their Torah and Injeel (Gospel) but it does not mean that they found his description at the time of Moosaa . Taahir Ibn 'Aashoor interpreted the above two verses saying: 'Allaah's Mercy would also include those who believed in Allaah's Verses, and it means the future verses because the people believed in the verses revealed to Moosaa at his time. It refers to the Jews and Christians who were at the time when Muhammad was made a Prophet, and thereafter, as Allaah Says (what means): {…whom they find written [i.e. mentioned] in what they have of the Torah and the Gospel.}, and as Allaah further Says (what means): {…and relieves them of their burden and the shackles which were upon them.} This means that the legislation which was revealed to them was difficult, and Allaah's Verses refers to the Quran, as the words of the Quran are called verses because it was an obstacle for the fluent and eloquent Arabs to challenge them, and they (the verses) are evidence that they are from Allaah and that His Messenger is truthful.' For more benefit about the glad tidings which were mentioned in the Torah and the Gospel about the advent of Muhammad please refer to Fatwa 83990. As for the method of debating with the Christians and calling them to Islam, please refer to Fatwa 84349.

Finally, it should be noted that a Muslim should preserve his religion from misconceptions, and he should not listen to them as a misconception could find its way to his heart and thus becomes very difficult for him to repel it because of his weak faith or his lack of knowledge or for both reasons. Indeed the scholars stated that it is forbidden to read the books of the people of the book because they were distorted and falsified, so how would one listen to their misconceptions, as this is more forbidden? For more benefit, please refer to Fataawa 83344 and 88276. 

Allaah Knows best.