Had John been cast into prison when Jesus began His ministry?

Answer 1

The New Testament Synoptic gospels are all in agreement that John the Baptist was imprisoned early in Jesus' ministry, while John places his imprisonment a little later (John 3:24). However, the Jewish historian, Flavius Josephus, identifies his imprisonment as being because of John's objection to his marriage to Herodias, which therefore places John's imprisonment no earlier than approximately 36 CE. (1)
Either the ministry of Jesus began much later than commonly believed, or the gospels were in error on this point.
Alternatively, Josephus was himself in error and the gospels correct. However, Mark (6:17) and Matthew (14:3) do agree with Josephus that John was imprisoned because of Herodias, and the approximate date of the marriage is well-attested.

Evidence from Josephus (Antiquities of the Jews Book XVIII)
Josephus places the marriage of Antipas to his brother's former wife, Herodias, at the same time as, or shortly after, the death of Philip II, who he says died in the twentieth year of the reign of Tiberius (34 CE). John was highly critical of this marriage, causing Antipas to fear that he would cause an insurrection:

"Herod, who feared lest the great influence John had over the people might put it into his power and inclination to raise a rebellion, (for they seemed ready to do any thing he should advise) thought it best, by putting him to death, to prevent any mischief he might cause, and not bring himself into difficulties, by sparing a man who might make him repent of it when it would be too late. Accordingly he was sent a prisoner, out of Herod's suspicious temper, to Macherus, the castle I before mentioned, and was there put to death."

Josephus makes it clear that Antipas intended to execute John, and it seems most improbable that he would long delay the execution if he feared an insurrection by John's supporters. The execution would have quickly followed the imprisonment.

Additionally, King Aretas attacked and defeated Antipas in 36 CE in revenge for the slight to his daughter, whom Antipas divorced in order to marry Herodias. We can expect a one or two year delay during which the news reached Aretas and he considered a diplomatic solution or demanded compensation, then raised an army, but an angry father is not likely to have waited ten years to take action. Some of the Jews believed that Antipas' defeat was divine retribution for his execution of John: Now the Jews had an opinion that the destruction of this army was sent as a punishment upon Herod, and a mark of God's displeasure to him.

This association makes it most likely that John's death was a quite recent event at the time of the defeat - closer in time to the battle than to the wedding, and certainly not something that occurred 8 or 10 years earlier.