In the fourth age. - An age here means the average period from the birth to the death of one man. This use of the word is proved by Numbers 32:13 - "He made them wander in the wilderness for forty years, until all the generation that had done evil in the sight of the Lord was consumed." This age or generation ran parallel with the life of Moses, and therefore consisted of one hundred and twenty years. Joseph lived one hundred and ten years. Four such generations amount to four hundred and eighty or four hundred and forty years. From the birth of Isaac to the return to the land of promise was an interval of four hundred and forty years. Isaac, Levi, Amram, and Eleazar may represent the four ages.

For the iniquity of the Amorite is not yet full. - From this simple sentence we have much to learn. First. The Lord foreknows the moral character of people. Second. In his providence he administers the affairs of nations on the principle of moral rectitude. Third. Nations are spared until their iniquity is full. Fourth. They are then cut off in retributive justice. Fifth. The Amorite was to be the chief nation extirpated for its iniquity on the return of the seed of Abram. Accordingly, we find the Amorites occupying by conquest the country east of the Jordan, from the Arnon to Mount Hermon, under their two kings, Sihon and Og Numbers 21:21-35. On the west of Jordan we have already met them at En-gedi and Hebron, and they dwelt in the mountains of Judah and Ephraim Numbers 13:29, whence they seem to have crossed the Jordan for conquest Numbers 21:26. Thus had they of all the tribes that overspread the land by far the largest extent of territory. And they seem to have been extinguished as a nation by the invasion of Israel, as we hear no more of them in the subsequent history of the country.

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.

And in the fourth generation they shall come hither again,.... The seed of Abram were in the land of Canaan before their descent into Egypt; and it is here predicted and promised, that they should come thither again, as they did, in the fourth generation of those that descended thither; for Moses and Aaron were the fourth from Levi, or Eleazar from Kohath, and Caleb from Judah; or rather this was in the fourth age or century from the birth of Isaac, when the four hundred were up before mentioned, men living at that time about an hundred years: Grotius interprets this of the fourth generation of the Amorites, because of what follows:

for the iniquity of the Amorites is not yet full; and therefore as yet would not be turned out of the land, and the seed of Abram could not till then inherit it: wicked people have a measure of iniquity to fill up, which is known of God; some are longer, some are quicker in filling it up, during which time God waits patiently and bears with them; but, when it is completed, he stays no longer, but takes vengeance on them, Matthew 23:32. The Amorites were only one of the nations of the Canaanites, but were a very strong and powerful one, and are put for them all, and are the rather mentioned, because Abram at this time dwelt among them; and it seems as if there were some good men among them, such as the confederates of Abram might be, and they were not arrived to that depth of wickedness they afterwards would and did, and which brought on their ruin, and so made way for the posterity of Abram to inherit their land. Ben Melech interprets it of the punishment of the sin of the Amorites, the time for that was not come to dispossess them of their land.

But in the fourth generation they shall come hither again: for the {e} iniquity of the Amorites is not yet full.

(e) Though God tolerates the wicked for a time, yet his vengeance falls on them when the measure of their wickedness is full.

15:16 They shall come hither again - Hither to the land of Canaan, wherein thou now art. The reason why they must not have the land of promise in possession till the fourth generation, is because the iniquity of the Amorites was not yet full. The righteous God has determined, that they shall not be cut off till they are arrived to such a pitch of wickedness; and therefore till it come to that, the seed of Abram must be kept out of possession.

15:12-16 A deep sleep fell upon Abram; with this sleep a horror of great darkness fell upon him: a sudden change. The children of light do not always walk in the light. Several things were then foretold. 1. The suffering state of Abram's seed for a long time. They shall be strangers. The heirs of heaven are strangers on earth. They shall be servants; but Canaanites serve under a curse, the Hebrews under a blessing. They shall be suffers. Those that are blessed and beloved of God, are often sorely afflicted by wicked men. 2. The judgment of the enemies of Abram's seed. Though God may allow persecutors and oppressors to trample upon his people a great while, he will certainly reckon with them at last. 3. That great event, the deliverance of Abram's seed out of Egypt, is here foretold. 4. Their happy settlement in Canaan. They shall come hither again. The measure of sin fills gradually. Some people's measure of sin fills slowly. The knowledge of future events would seldom add to our comfort. In the most favoured families, and most happy lives, there are so many afflictions, that it is merciful in God to conceal what will befall us and ours.