In earlier days, John had rebuked the common people as well as the religious leaders (cf. Luke 3:7ff). Of course, there was little danger to himself personally by issuing these rebukes. However, such was not the case regarding Herod who had the power of life and death in his hand. The people would naturally wonder if John would rebuke Herod for his adultery (and other sins) or be quiet to keep his own life safe. Many in John's day probably thought him to be a fool to deliver this rebuke, knowing that his liberty or life would probably be taken. Had he asked for advice, many would have encouraged John not to deliver this rebuke but to serve God in other ways and be useful for years to come. However, John understood that no man is worthy to stand before the people and call them to repentance as he had done, if he showed partiality by ignoring sin in high places. May more of today's preachers come to this realization! There will be times when one is tempted to only preach the smooth and easy parts of God's word to avoid confrontation. However, a faithful servant of the Lord will preach all of God's word, even when it is uncomfortable or dangerous to do so (II Tim. 4:2). Although it is unlikely that one today will literally lose his head as John did, one should expect persecution for standing firm for the way of righteousness (e.g., loss of a job, ridicule, etc.; cf. II Tim. 3:12).
It should also be observed that there is no record of John telling Herod to be baptized in order to remedy this sin. John "preached a baptism of repentance for the remission of sins" (Luke 3:3), but Herod could not have been classified as one who repented until he severed his "marriage" to Herodias (technically, it was adultery). Then, and only then, would John's baptism have done him any good. Although John's baptism is no longer in force (cf. Acts 19:1-5), the same principle is still true today under the baptism that Christ commanded in Matthew 28:19,20. Baptism still requires repentance (cf. Acts 2:38). Baptism does not change a sinful action or relationship into a holy one. If one is living as a bank robber, baptism will not correct the problem until a change in attitude and action is manifested. One must stop robbing banks if he is to be forgiven. Likewise, if one is living in adultery, baptism will not correct the problem (cf. Matt. 19:9). One must stop committing adultery if he is to be forgiven. Therefore, the first step in remedying this sin is the severing of the illicit relationship, which is a proof of repentance. It should be emphasized that although Antipas had married Herodias in the eyes of man, she was still Philip's wife according to John (and according to God)! Thus, merely because a civil authority says that a marriage is valid does not mean that God concurs!
In addition to correcting Herod regarding his adulterous relationship with Herodias, John also issued rebukes concerning "all the evils which Herod had done" (Luke 3:19). Herodias became so enraged at John that she would've killed him if she had only had the means and opportunity. The thought probably never crossed her mind that John was trying to help her. She would rather kill the "doctor" and ignore his "prescription." It is possible that she became so enraged because she understood that if Herod heeded his advice, she would be a ruined woman. She would be dethroned and disgraced with nowhere to go.
Herod did not have the same passion against John as Herodias did. In fact, "Herod feared John, knowing that he was a just and holy man, and he protected him" (Mark 6:20). Herod feared John too much to kill him, but not enough to keep from imprisoning him to appease Herodias. There were times when Herodias' influence on Herod was strong and the only thing that saved John's life was Herod's fear of the multitudes (cf. Matt. 14:5).
Ironically, Herod feared John enough to listen to him speak on various occasions. In fact, Mark records that Herod heard John "gladly." Men are often willing to do many things that God commands but are unwilling to give up some particular sin. Herod evidently obeyed John's teaching on certain matters, but he wouldn't give up his adulterous "marriage." It is still true today that the sins that are dearest to us are the ones that truly test the strength of our love for God. Will we forsake them or Him?