“commits adultery against her.” The context of this statement that Jesus made to his disciples is the Pharisees’ question to him, which is partially given in Mark 10:2, but fully given in Matthew 19:3 (see commentary on Mark 10:2 and commentary on Matt. 19:3). The Pharisees were asking Jesus about the teaching of the school of Hillel, that a man could divorce his wife for any reason at all. The social context and the question are specific, and the reason Jesus answered the way he did is explained in Matthew (see commentary on Matthew 19:9).
We should immediately notice that Mark is giving us an abbreviated version of Jesus’ comments when we compare Jesus answer here and in Matthew 19:9. At least in Matthew 19 Jesus seems to allow for divorce for the case of sexual immorality, but he leaves that out here. This shows us that we are to understand Jesus’ answer in light of the full question and debate, not just grab onto Jesus’ short answer here in Mark and try to run our lives by it as if it was the whole truth of the situation.
Mark leaves out the part of the event that is the answer Jesus gives directly to the Pharisees, and moves forward to something Matthew does not cover: the disciples being in the house and asking Jesus again about the subject. The rabbinic debate and social context of the disciples’ question is well understood and defined by Pharisee’s beliefs and actions and also by the record of the event in the Gospel of Matthew. That is why Mark only needs to record Jesus giving a very short, possibly abbreviated, answer to the disciples’ question.
The disciples knew that the Pharisees were not divorcing their wives because of sexual immorality. The Pharisees were champions of easy divorce and wanted Jesus’ opinion on it, which is why they asked him about divorce in the first place (Matt. 19:3; Mark 10:2). So Jesus was answering the disciples’ question in a well-known social situation: some of the Pharisees were divorcing their wives just to be with other women, and in the culture, although it was rare, some women were divorcing their husbands just to be with other men. Josephus records that Salome, wife of Costobarus, divorced him (Antiquities 15.7.10); and we know that Herodias divorced her husband Philip and married Herod Antipas ruler of Galilee. John the Baptist confronted Herod Antipas about his marriage to Herodias, which is why he ended up in jail and then executed (Mark 6:17-28).
We are now in a position to understand Jesus’ very short answer to his disciples about divorce and remarriage. In the context of people such as the Pharisees getting divorced simply because they liked someone else better than their spouse, the answer Jesus gave his disciples in Mark 10:11-12 is certainly correct. Divorcing someone for no other reason than you like someone better than your spouse is in effect the same as simply having an affair with that other person—you are committing adultery.
We should note that technically, Mark 10:11-12 are similar to Matthew 19:9 in that the phrase “commits adultery” is actually a passive verb in the Greek text. The verb is moichaō (#3429 μοιχάω, pronounced moy-kah’-ō), and it is in the passive voice. The passive verb is very important for the interpretation of the verse in Jesus’ Sermon on the Mount where the wife is clearly a victim (see commentary on Matthew 5:32), but not so important here or in Matthew 19:9, because in these verses the wicked husbands and wives are both the agent and the subject of the verb. They are the “victim” of their own action. Thus, while it is true that the husband or wife “is made to commit adultery,” he or she was made adulterous by their own action of divorcing and remarrying another person. Nevertheless, a technically correct translation of the phrases would be “is made adulterous,” instead of “commits adultery.” The person makes themselves adulterous by their own action.