John Calvin Commentary Psalms 63:11

"But the king shall rejoice in God: Every one that sweareth by him shall glory; For the mouth of them that speak lies shall be stopped." — Psalms 63:11 (ASV)

But the king will rejoice in God. The deliverance David received was not extended to him as a private individual, but the welfare of the whole Church was involved in it, just as the welfare of the body depends on the safety of the head. Therefore, it is fitting for him to represent all the people of God as rejoicing with him.

Nor can we fail to admire his holy magnanimity in not hesitating to call himself king, overwhelming as the dangers were that surrounded him, because he laid claim to that honor by faith, though he was still denied it in actual possession. In saying that he would rejoice in God, he refers to the gratitude he would feel; at the same time, in extolling the divine goodness shown to him, he views it as affecting the whole body of the faithful.

As was already remarked, the safety of God’s chosen people, at that time, was inseparably connected with the reign of David and its prosperity—a figure by which God intended to teach us that our happiness and glory depend entirely on Christ. By those who swear in the name of the Lord, he generally means all His genuine servants.

The act of solemnly calling on God to witness and judge what we say is one part of divine worship; hence an oath, by the figure of speech called synecdoche, signifies the profession of religion in general. We should not imagine from this that God considers all those to be His servants who merely mention His name.

Many take His name upon their lips only to profane it with the grossest perjury; others outrage or slight His name by making trivial and unnecessary oaths; and hypocrites are guilty of wickedly abusing it. But those to whom David refers are those who swear by the Lord thoughtfully and with reverence, and whose hearts respond to what they declare.

This appears more clearly from the contrast that follows in the verse, where he opposes those who swear by the name of God to those who speak lies, understanding by that term not only treacherous and deceitful men, but also men who profane the name of God with sacrilegious falsehoods.