John Calvin Commentary Acts 2:30

"Being therefore a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins he would set [one] upon his throne;" — Acts 2:30 (ASV)

Therefore, seeing he was a prophet. He shows, by two reasons, that it is no wonder if David speaks of things that would come to pass long after his time; the first is, because he was a prophet. And we know that future things, and those far removed from human knowledge, are revealed to the prophets.

Therefore, it would be wickedness to measure their speeches according to the common manner and order which we use in measuring the speeches of other men, since they go beyond long periods of time, having the Spirit as their director. For this reason, they are also called seers; because, being placed, as it were, on a high tower, they see those things which, by reason of great distance, are hidden from other men.

Another reason is that Christ was promised to him specifically. This maxim was so common among the Jews that they frequently had the son of David on their lips whenever Christ was mentioned.

These are not arguments, I confess, that necessarily prove this prophecy should be interpreted as referring to Christ; nor was that Peter’s intent and purpose. Instead, he first meant to counter the objection as to how David had such skill to foretell something unknown. Therefore, Peter says that David knew Christ, both by prophetic revelation and also by a special promise.

Furthermore, this principle was very influential among the more discerning people, as Paul states, Christ is the end of the law (Romans 10:4). Therefore, no one doubted that this was the goal at which all the prophets aimed: to lead the godly to Christ, as it were, by the hand.

Therefore, whatever notable or extraordinary thing they uttered, the Jews were commonly persuaded that it applied to Christ. Furthermore, we must note that Peter reasons soundly when he inferred that David was not ignorant of that which was the most important point of all revelations.

He had sworn with an oath. God swore not only so that he might make David believe his promise, but also so that the thing promised might be held in greater esteem. And for this purpose, in my judgment, it is repeated here, so that the Jews might reflect on how weighty the promise was, which God made so notable (and so famous). This same admonition is also profitable for us.

For we need not doubt that the Lord meant to set forth the excellence of the covenant by adding a solemn oath. Meanwhile, this is also a suitable remedy for the weakness of our faith: that the sacred name of God is presented to us, so that his words may carry greater credibility.

These words, according to the flesh, declare that there was something more noble in Christ than the flesh. Therefore, Christ came from the seed of David as a man in such a way that he nevertheless retains his divinity. Thus, the distinction between the two natures is plainly expressed, just as Christ is called the Son of God according to his eternal essence, in the same way that he is called the seed of David according to the flesh.