If the Quran is true, then Islam is false.  Why?  Because the Quran says to trust the Gospels and the four Gospels contradict the Quran. This means the Quran can’t be true if it says the gospel message is also true.  In order to show this, I will quote several verses from the Quran.  The more important ones are bolded for easy reference later.

1. None can change Allah’s words
   1. 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.”
   2. 6:115, “The word of thy Lord doth find its fulfillment in truth and in justice: None can change His words: for He is the one who heareth and knoweth all.”
   3. 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.”
2. Moses was given the Torah
   1. 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!
   2. 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).”
3. David was given the Psalms
   1. 4:163, “We have sent thee inspiration, as We sent it to Noah and the Messengers after him: we sent inspiration to Abraham, Isma’il, Isaac, Jacob, and the Tribes, to Jesus, Job, Jonah, Aaron, and Solomon, and to David, We gave the Psalms.”
4. Jesus was given the gospel
   1. 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.
5. Christians should judge by what is in the Gospel
   1. 5:47, “Let the people of the Gospel judge by what Allah hath revealed therein. If any do fail to judge by (the light of) what Allah hath revealed, they are (no better than) those who rebel.”
6. The Quran confirms the Torah, the Psalms, and the Gospels and guards them
   1. 5:48 To thee [the people of the gospel] We sent the Scripture in truth, confirming the scripture that came before it, and guarding it in safety: so judge between them by what Allah hath revealed, and follow not their vain desires, diverging from the Truth that hath come to thee. To each among you have We prescribed a law and an open way. If Allah had so willed, He would have made you a single people, but (His plan is) to test you in what He hath given you: so strive as in a race in all virtues. The goal of you all is to Allah; it is He that will show you the truth of the matters in which ye dispute;
7. Ask Christians about spiritual things
   1. 10.94, “If thou were 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.”
8. Curse on those who say God has a son
   1. 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!
9. Jesus not crucified
   1. 4:157, “That they said (in boast), “We killed Christ Jesus the son of Mary, the Messenger of Allah”;- but they killed him not, nor crucified him, but so it was made to appear to them, and those who differ therein are full of doubts, with no (certain) knowledge, but only conjecture to follow, for of a surety they killed him not.”

Alright, so we see that…

1. None can change Allah’s words (6:34; 6:115; 18:27).
2. Jesus was given the gospel by Allah (57:27).
3. The Quran confirms the Gospels (5:48)
4. The Gospels contradict the Quran

So, if the Quran confirms the Gospels which says that Jesus was crucified (Matthew 27:26–31; Mark 15:15–20; Luke 23:32–34; John 19:17-18) and the Quran says Jesus was not crucified (Surah 4:157), then the Quran is false.  But if it says the Gospels are false, then the Quran denies its own teaching and Islam is false.

Answering objections in defense of the Quran

Objection:  The gospel is not the written record, but the message that Jesus gave.

This can’t work because the Quran confirms the Torah, the Psalms, and the gospels (written records) that were around at the time of Muhammad.

Surah 5:48 To thee [the people of the gospel] We sent the Scripture in truth, confirming the scripture that came before it, and guarding it in safety…