Mark 6:18

“For John had said unto Herod, etc..] He had been personally with him, and had plainly and freely told him of his sin, and faithfully reproved him for it; alleging what he had done was contrary to the law of God: it is not lawful, for thee to have thy brother's wife: (see Gill on “<401404>Matthew 14:4”).”

“See this account of the death of John the Baptist fully explained in the notes at Matthew 14:1-12. Mark 6:20 For Herod feared John - That is, he stood in awe of him on account of his sanctity, and his boldness and fearlessness in reproving sin. Knowing that he was a just man and an Holy - A holy, pious, upright, honest man - a man who would not be afraid of him, or afraid to speak his real sentiments. And observed him - Margin, “kept him, or saved him.” This does not mean that he “observed” or obeyed his teachings, but that he kept him in safe custody in order to preserve him from the machinations of Herodias. He was willing to show his respect for John, and to secure him from danger, and…”

“6:18 Mark gives the moral-religious reason for the imprisonment and execution of John the Baptist. Josephus refers to the political reason (fear that John’s great popularity might start a revolution; Josephus, Antiquities 18.5.2). Similarly, the religious reasons for Jesus’ condemnation (Mark 14:63-64) became political when he was brought before Pontius Pilate (15:2; Luke 23:2-3). • It is against God’s law: Not only was the relationship adulterous (Lev 20:10), but it violated the further law against marrying a brother’s wife (Lev 18:16; 20:21).”

“It is: Lev 18:16, Lev 20:21; Kg1 22:14; Eze 3:18, Eze 3:19; Mat 14:3, Mat 14:4; Act 20:26, Act 20:27; Act 24:24-26 Mark 6:19”