Catechism of the Catholic Church

In his preaching Jesus unequivocally taught the original meaning of the union of man and woman as the Creator willed it from the beginning: permission given by Moses to divorce one's wife was a concession to the hardness of hearts. The matrimonial union of man and woman is indissoluble: God himself has determined it: "what therefore God has joined together, let no man put asunder." (2336, 2382)