Genesis 15:16 - Clarke's commentary and critical notes on the Bible

But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full. In the fourth generation - In former times most people counted by generations, to each of which was assigned a term of years amounting to 20, 25, 30, 33, 100, 108, or 110; for the generation was of various lengths among various people, at different times. It is probable that the fourth generation here means the same as the four hundred years in the preceding verse. Some think it refers to the time when Eleazar, the son of Aaron, the son of Amram, the son of Kohath, came out of Egypt, and divided the land of Canaan to Israel, Joshua 14:1. Others think the fourth generation of the Amorites is intended, because it is immediately added, The iniquity of the Amorites is not yet full; but in the fourth generation they should be expelled, and the descendants of Abram established in their place. From these words we learn that there is a certain pitch of iniquity to which nations may arrive before they are destroyed, and beyond which Divine justice does not permit them to pass.