John Chrysostom — as excerpted in the Catena Aurea on Matthew 19:1-8

“When He had brought forward the words and facts of the old law, He then interprets it with authority, and lays down a law, saying, Therefore they are no more twain, but one flesh. For as those who love one another spiritually are said to be one soul, And all they that believed had one heart and one soul, (Acts 4:32.) so husband and wife who love each other after the flesh, are said to be one flesh. And as it is a wretched thing to cut the flesh, so is it an unjust thing to put away a wife.”