Psalms 77:15 - Matthew Poole's English Annotations on the Holy Bible

Redeemed thy people, to wit, out of Egypt, after a long and hard bondage; which he here mentions to strengthen his faith in their present captivity. Jacob and Joseph; whom he mentions, partly, as a most eminent portion of the sons of Jacob, branched forth into two numerous tribes; partly, because the sons of Joseph were born in Egypt, which Jacob's other sons were not; and partly, because he laid the foundation of that redemption by bringing them into Egypt, and preserving and nourishing Jacob and his sons there, as a little child is nourished, as it is expressed in the Hebrew text, Genesis 47:12; in which respect he was a second father to them, and they might well be called his sons; without whose care (to speak humanly) there had been no such redemption, nor people to be redeemed.