If the quran is true, then islam is false. Why? Because the quran says to trust the Gospels, but the four Gospels contradict the quran. This means the quran can’t be true if it says the gospel message is also true.
None can change allah’s words
6:34, “Rejected were the messengers before thee: with patience and constancy they bore their rejection and their wrongs, until Our aid did reach them: there is none that can alter the words (and decrees) of allah. Already hast thou received some account of those messengers.”
6:115, “The word of thy Lord doth find its fulfilment in truth and in justice: None can change His words: for He is the one who heareth and knoweth all.”
18:27, “And recite (and teach) what has been revealed to thee of the Book of thy Lord: none can change His Words, and none wilt thou find as a refuge other than Him.”
Moses was given the Torah
2:87, “We gave Moses the Book and followed him up with a succession of messengers; We gave Jesus the son of Mary Clear (Signs) and strengthened him with the holy spirit. Is it that whenever there comes to you a messenger with what ye yourselves desire not, ye are puffed up with pride?–Some ye called impostors, and others ye slay!”
3:3, “It is He Who sent down to thee (step by step), in truth, the Book, confirming what went before it; and He sent down the Law (of Moses) and the Gospel (of Jesus) before this, as a guide to mankind, and He sent down the criterion (of judgment between right and wrong).”
Jesus was given the gospel
57:27, “Then, in their wake, We followed them up with (others of) Our messengers: We sent after them Jesus the son of Mary, and bestowed on him the Gospel; and We ordained in the hearts of those who followed him Compassion and Mercy. But the Monasticism which they invented for themselves, We did not prescribe for them: (We commanded) only the seeking for the Good Pleasure of allah; but that they did not foster as they should have done. Yet We bestowed, on those among them who believed, their (due) reward, but many of them are rebellious transgressors.”
Ask Christians about spiritual things
10:94, “If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt.”
Curse on those who say God has a son
9:30, “The Jews call 'Uzair a son of Allah, and the Christians call Christ the son of allah. That is a saying from their mouth; (in this) they but imitate what the unbelievers of old used to say. allah’s curse be on them: how they are deluded away from the Truth!”
If the quran confirms the Gospels which say that Jesus was crucified and the quran says Jesus was not crucified (surah 4:157), then the quran is false. But if it says that the Gospels are false, then the quran denies its own teaching, and islam is false.