Thy seed shall be strangers — So they were in Canaan first, Psalms 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, Exodus 1:13. See how that which was the doom of the Canaanites, Genesis 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 Exodus 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, Genesis 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.
Related Commentaries of Genesis 15:13
Genesis 15:13
13 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 four hundred years;