And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them {d} four hundred years;

(d) Counting from the birth of Isaac to their departure of Egypt: Which declares that God will allow his to be afflicted in this world.

Wesley's Notes

15:13 Thy seed shall be strangers - So they were in Canaan first, Psa 105:11,12, and afterwards in Egypt: before they were lords of their own land, they were strangers in a strange land. The inconveniences of an unsettled state make a happy settlement the more welcome. Thus the heirs of heaven are first strangers on earth. And them they shall serve - So they did the Egyptians, Exo 1:13. See how that which was the doom of the Canaanites, Gen 9:25, proves the distress of Abram's seed: they are made to serve; but with this difference, the Canaanites serve under a curse, the Hebrews under a blessing. And they shall afflict them - See Exo 1:11. Those that are blessed and beloved of God are often afflicted by wicked men. This persecution began with mocking, when Ishmael the son of an Egyptian, persecuted Isaac, Gen 21:9, and it came at last to murder, the basest of murders, that of their new born children; so that more or less it continued 400 years.

Matthew Henry's Concise Commentary

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.