That's a good point, but there's no claim in any religious writ other than the Quran that what is being read is from god, but rather an account of the works of god. I may be wrong but even the Jewish prophets were only writing in human tongue what Yaw supposedly passed on to them through revelation. The NT especially consists of letters and commentaries directed to specific groups. And the followers of the four gospels were at odds with one another. Iraneous was revolutionary in saying there was nothing wrong with worshipping more than one gospel. Then you have to consider why an almighty god would rely on the fragility of the written word, why 'his word' is written in numerous languages, etc. If only the NT is true, why does Jude and Paul quote heavily from the Book of Enoch? What of the Catholic apocrypha? What of the evidence that the authorship of the NT is pseudonymous? If one relies on what god says, then only Christianity is true, only Judaism is true, only Islam is true. To say nothing of lesser Abrahamic faiths - Mormonism, Jehovah's Witnesses, Rasta, etc.
Also, that quote from St. Cyril of Jerusalem concens what is now called the Infancy of Thomas and though its not at all historical there's really no evidence that it was authored by Mani or Marcion or Valentinus or any of the Orthodox boogy-men. As opposed to the Gospel of John, which is thoroughly gnostic and says...
And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen.