John the Baptist is one of very few New Testament figures independently described by a non-Christian source, and the description does not match the Gospels' framing.
Josephus, writing his Jewish Antiquities in the nineties, reports that Herod Antipas executed John, called the Baptist, a good man who exhorted the Jews to virtue and to justice toward one another and piety toward God, and whose baptism was for the purification of the body once the soul had already been cleansed by righteousness. Josephus says Herod feared that John's influence over the crowds might lead to an uprising and struck first, at the fortress of Machaerus. He adds that some Jews regarded the destruction of Herod's army in a later war as divine punishment for the killing.
The passage is important for two reasons. Unlike the celebrated reference to Jesus in the same work, this one is not suspected of Christian tampering; it is written in Josephus' own terms and gives John an interpretation of baptism that no Christian would have inserted. And its motive is political, crowd control, where the Gospels give a domestic motive, John's denunciation of Herod's marriage to Herodias, formerly his brother's wife. The two are not incompatible, but they are not the same story, and Josephus is the earlier witness by decades in composition terms.
That John existed, baptised, gathered a following, and was executed by Antipas is about as secure as anything in first-century Judaean religious history.