From The Gospel of Mark, by Mary Healy, commenting on Mark 10:2-16
Now Jesus comes to the heart of the matter, the real “commandment” he wishes to draw attention to, which is given not in the fifth book of Moses, Deuteronomy, but in the first, Genesis. He quotes two passages from the story of creation, referring to humanity prior to the sin of Adam and Eve. The first, Gen 1:27, recounts God’s creation of human beings in his image on the sixth day: God made them male and female. The second, Gen 2:24, describes the covenant bond of love between husband and wife, expressed in sexual union: a man shall leave his father and mother [and be joined to his wife], and the two shall become one flesh.
In biblical thought flesh is not merely the physical body but the whole human being as present in the visible world. For a husband and wife to become “one flesh” is the bodily expression of a personal union at the deepest level of their being. Jesus links these two scriptures to indicate that the communion of love between a husband and wife is a sign pointing to God’s ultimate purpose in creating humanity in his image.
With these statements, Jesus brings the discussion—and the whole understanding of marriage itself—to a new level. By the very fact of referring to humanity before the fall, Jesus is implying that from now on, God’s original intention is the true standard for marriage and other human relationships. He is saying, in effect, that the concession in Deuteronomy no longer applies because humanity is no longer captive to sin, hardness of heart, and the resultant family breakdown. Now there is a new reality at hand—the kingdom of God—with a new power to live and experience what God intended from the beginning. As Jesus has already suggested (Mark 8:31–9:1), this new possibility will come about through his paschal mystery.
Jesus concludes with his own solemn injunction: what God has joined together, no human being must separate. He thereby confirms what Genesis already implied: the union of husband and wife is no mere human convention but a bond made by God himself (see Mal 2:14–16). No human being is authorized to dissolve that bond once it has been made.