There is more evidence for the New Testament writings than for many writings of classical authors, the authenticity of which no one dreams of questioning. If the NT were a collection of secular writings there would be no doubt to their authenticity. They would be beyond doubt. In fact, Historians have been much readier to trust the NT records than have many Theologians.
There are in existence over 5000 Greek manuscripts of the NT in whole or in part. The best and most important go back to somewhere around 350 AD.
We can appreciate how wealthy the NT is in manuscript attestation if we compare the textual material for other ancient historical works.
For Caesar's Gallic War, (composed between 58 and 50 BC) there are several extant mss, but only 9 or 10 are good, and the oldest is some 900 years later than Caesar's day.
Of the 142 books of the Roman History of Livy (59 BC-AD 17) only thirty five survive; these are known to us from not more than 20 mss of any consequence, only one of which, and that containing fragments of books iii-vi, is as old as the 4th century.
Of the 14 books of the Histories of Tasitus(AD-100) only 4 and one half survive; of the 16 books of his Annals, 10 survive in full and 2 in part. The text of these extant portions of his two great historical works depends entirely on 2 mss, one of the ninth century and one of the 11th.
The history of Thucydides (460-400 BC) is known to us from 8 mss, the earliest belonging to AD 900 and a few papyrus scraps, belonging to about the beginning of the Christian era.
The same is true about the History of Herodotus (488-428 BC).
Yet no classical scholar would listen to an argument that the authenticity of Herodotus or Thucydides is in doubt because the earliest mss of their works which are of any use to us are over 1,300 years later than the originals.
But how different is the situation of the New Testament in this respect!
In addition to the 5000 Greek manuscripts mentioned, there are fragments of papyrus copies of books of the NT dated from 100-200 years earlier still.The Chester Beatty Biblical Papyri, the existence of which was made public in 1933, consist of portions of 11 papyrus codices, 3 of which contain most of the NT writings.
There is extensive evidence, numerous mss, of which are readily available to anyone seeking as such. Most of my info comes from the writings of FF Bruce (1910-1990). I have just begun reading this field and have yet to scratch the surface.