What does Mark 6:20 mean?
Herod Antipas' father, Herod the Great, was Edomite but raised Jewish. Antipas' mother was a Samaritan. He was installed as ruler over Galilee by the Roman emperor. Antipas is in the strange position of ruling over people he somewhat understands, but who despise everything he is and represents.Antipas knows John is a righteous man and he fears him, plus the tetrarch likes to listen to John. So since Herodias is trying to kill John, Antipas holds him in the one place she can't reach: prison. Imprisonment of the offender should also, presumably, appease his wife.
Josephus writes that Antipas initially refused to put John the Baptist to death for fear that John's followers would revolt. The tradition of Jews rising up against their Roman oppressors was a long one, and the mythology of the coming Messiah who will succeed seemed to be consistently on the Jews' minds. In addition, Antipas had divorced the daughter of the king of Nabatea, which borders his southern territory of Perea. Antipas would't be able to defend Nabatea from Perea and Galilee from its Jewish citizens, so this line of reasoning makes sense, and Matthew affirms it (Matthew 14:5). Josephus wrote his account sixty years after the events for a Roman audience. Romans would have understood the fear of invasion and rebellion more readily than the idea that Antipas was actually interested in what John had to say or that he feared John.
Despite the appeal of John's words, Antipas can't get past the stage of being puzzled. He understands that John's words are true and right, but they don't get into Antipas' heart. Later, Antipas' great-half-nephew Herod Agrippa II will have a similar reaction to Paul (Acts 26:1, 24–32). And many people today treat Jesus the same. They follow Pilate's example and consider Him a "good teacher," a good man with good things to say (Mark 15:5, 14). But they cannot accept the purpose of Jesus' message. The gospel is not designed to teach us how to act so we have peace and success in this world. It is to show us how to submit to God and gain eternal life with Him. John challenges Antipas to follow the Mosaic Law regarding marriage to a sister-in-law. Jesus calls us to love others and treat each other kindly. These are good and right things but have no eternal value without repentance and trust in Christ.