What does Luke 3:20 mean?

When Herod the Great died (Matthew 2:19), his territory was split up among three of his sons. Two of these were Herod and Philip (Luke 3:1). At one point, Philip was married to a woman named Herodias. When Herod divorced his wife, Herodias divorced Philip and married Herod. This was against the Law of Moses (Leviticus 18:16; 20:21). John the Baptist condemned this as an illegitimate relationship (Luke 3:19), which likely offended Herod. He had John imprisoned—an act Luke wryly describes as yet another addition to Herod's long list of sins.

While Herod had a vague respect for John the Baptist (Mark 6:20), his wife Herodias was much more spiteful. She seems to have been the one pressing for John's arrest (Mark 6:17–19). Later, she would use her own daughter to manipulate Herod, resulting in John's gruesome execution (Mark 6:22–27).