Pliny governed Bithynia and Pontus for Trajan and corresponded with him about the ordinary problems of the province. One of those problems was what to do about people denounced as Christians, and his letter about it survives with the emperor’s reply.
What it says
That people were brought before him on accusation rather than sought out; that he tested them by requiring sacrifice and reverence to the emperor’s image; that those who persisted were executed for obstinacy rather than for any specific crime he could identify; that some had abandoned the movement years before; and that they met before dawn on a fixed day, sang responsively to Christ as to a god, and bound themselves by oath not to commit theft, robbery or adultery, nor to break their word.
Trajan’s reply instructs him not to seek them out and not to accept anonymous accusations.
Why it is valuable, and why the value is limited
It is a Roman administrative document, hostile in tone and bureaucratic in purpose, written by someone with no interest in commending the movement. That makes it a genuine external witness to what Christians in Asia Minor were doing around 112 CE — the practice, not the beliefs, and not any event in the New Testament.
It does not corroborate any biblical text, and it postdates the letters it is usually read beside by a generation or more. What it establishes is that the situation those letters address was a real one, in the same provinces, within living memory.
How it is cited here
By Internet Archive identifier, which resolves to the item record and returns the title. The Loeb edition prints the Latin and the English on facing pages, which matters for a text this heavily quoted.