I believe the earliest reference to Jesus by a non-Christian historian was by Flavius Josephus, a First Century Jewish historian.
In his book, Antiquities, Josephus makes two references to Jesus. One of these is disputed by historians, but the other is accepted by almost everybody.
In this second passage, Josephus describes the trial and execution of St. James the Just, who he describes as “the brother of Jesus, who was called Christ.”
Josephus, Tacticus, Suetonius and Pliny the younger, Celsus, Lucian, Thallus have written things on Jesus or Nazareth. Even the Talmud Sanhedrin 43a mentions Christ’s influence.
I just finished a good book “The Christians as the Romans Saw Them” which reviews extant non-Christian literature to draw a picture of early Christianity from the viewpoint of prevailing culture of the day. I highly recommend it.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.