John Calvin Commentary Psalms 77:15

"Thou hast with thine arm redeemed thy people, The sons of Jacob and Joseph. Selah" — Psalms 77:15 (ASV)

You have redeemed your people by your arm. The Psalmist here celebrates, above all the other wonderful works of God, the redemption of the chosen people, to which the Holy Spirit everywhere throughout the Scriptures invites the attention of true believers, in order to encourage them to cherish the hope of their salvation.

It is well known that the power of God was at that time manifested to the Gentiles. The truth of history, indeed, through the artifice of Satan, was corrupted and falsified by many fables; but this is to be imputed to the wickedness of those in whose sight those wonderful works were performed, who, although they saw them, chose rather to blind their eyes and disguise the truth of their existence, than to preserve the true knowledge of them.

How can we explain the fact that they made Moses out to be some kind of magician or enchanter, and invented so many strange and monstrous stories, which Josephus collected in his work against Apion, except on the principle that it was their deliberate purpose to bury the power of God in forgetfulness?

It is not, however, so much the prophet's design to condemn the Gentiles for the sin of ingratitude, as to provide himself and other children of God with a basis for hope regarding their own circumstances. For at the time referred to, God openly exhibited, for the benefit of all future ages, a proof of his love toward his chosen people.

The word arm is used metaphorically here for extraordinary power, one worthy of remembrance. God did not deliver his ancient people secretly or in an ordinary way, but openly, and, as it were, with his arm stretched forth.

The prophet, by calling the chosen tribes the sons of Jacob and Joseph, assigns the reason why God accounted them as his people. This reason is the covenant he entered into with their godly ancestors. The two tribes which descended from the two sons of Joseph derived their origin from Jacob as well as the rest; however, Joseph's name is expressed to honor him, through whom the whole race of Abraham was preserved in safety.