John Calvin Commentary Psalms 78:70

"He chose David also his servant, And took him from the sheepfolds:" — Psalms 78:70 (ASV)

And he chose David his servant. After mentioning the temple, the prophet now proceeds to speak of the kingdom; for these two things were the chief signs of God’s choice of his ancient people, and of his favor towards them. Christ also has appeared as our king and priest to bring full and perfect salvation to us.

He proves that David was made king by God, who elevated him from the sheepfold and from the keeping of cattle to the royal throne. It serves greatly to magnify the grace of God that a peasant was taken from his humble shepherd’s cottage and exalted to the dignity of a king.

Nor is this grace limited to the person of David. We are taught that whatever worth there was in the children of Abraham flowed from the fountain of God’s mercy.

The whole glory and blessedness of the people consisted in the kingdom and priesthood, and both these are attributed to the pure grace and good pleasure of God. And it was necessary that the beginning of the kingdom of Christ should be lowly and contemptible, that it might correspond with its type, and that God might clearly show that he did not use external aids to accomplish our salvation.