I came across a reference to a letter from Pliny the Younger to the Roman emperor Trajan, asking his counsel on how he should deal with these Christians.
Pliny the Younger, not to be confused with his uncle Pliny the Elder–the important Roman naturalist and author who was killed by the volcano at Pompeii–was a lawyer, magistrate, and author who served as the governor of Bithynia and Pontus in present-day Turkey. In 112 A.D., he wrote the emperor, asking how he should proceed against the Christians, informing him about what he has been doing so far. The letter is the first mention of Christians in a pagan document. (For more background and a discussion of the letter, go here.)