Arcanum Divinae §6

⁶ This form of marriage, however, so excellent and so pre-eminent, began to be corrupted by degrees, and to disappear among the heathen; and became even among the Jewish race clouded in a measure and obscured. For in their midst a common custom was gradually introduced, by which it was accounted as lawful for a man to have more than one wife; and eventually when "by reason of the hardness of their heart,"(3) Moses indulgently permitted them to put away their wives, the way was open to divorce.