Many scholars believe that the 4 great Uncial Codices preserved to our day are the remnants of the 50 Greek New Testaments commissioned by Emperor Constantine for dissemination and edification of the churches. This involved using the finest prepared animal skins and copying the text by hand before sewing the quires into book form. An uncial codex is a text written in all capital letters and bound as a book of leaves as opposed to the former practice of recording texts on scrolls. These codices date from about the early to mid 4th century (325-335 C.E.). They are the earliest complete (or almost complete) witnesses to the New Testament. Though some papyri fragments preserved from Egypt are earlier (about 125 C.E.), these are only small portions of individual books instead of the whole N.T. Here is an article about finding one of these manuscripts from the dry desert of Sinai which aided its preservation. The other Great Codices are: Alexandrinus, Vaticanus, and Ephraemi.

Bible Treasure from a Desert Monastery – Part 2 | Patterns of Evidence