"Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery."
Jesus’ point was crystal clear—some things permitted in the Old Testament did not necessarily represent the ideal. Due to the hardness of ancient Israel’s heart, God tolerated (and regulated) some things under the Old Law that He did not endorse. As He did so, however, He progressively revealed His divine will to mankind, clarifying that will more fully through Christ.