John Calvin Commentary Psalms 21:7

"For the king trusteth in Jehovah; And through the lovingkindness of the Most High he shall not be moved." — Psalms 21:7 (ASV)

For the king trusteth. Here again the pious Israelites rejoice that their king will be established, because he relies on God; and they express at the same time how he relies on Him, namely, by hope or trust. I read the whole verse as one sentence, so that there is only one principal verb, and explain it as follows: The king, as he places by faith his dependence on God and His goodness, will not be subject to the disasters that overthrow the kingdoms of this world.

Moreover, as we have said before, whatever blessings the faithful attribute to their king belong to the whole body of the Church. A promise is made here, common to all the people of God, which may serve to keep us tranquil amid the various storms that agitate the world.

The world turns around, as it were, on a wheel, causing those who were raised to the very top to be precipitated to the bottom in a moment. But it is promised here that the kingdom of Judah, and the kingdom of Christ of which it was a type, will be exempt from such instability.

Let us remember that only those have the firmness and stability promised here who entrust themselves to the bosom of God by an assured faith and, relying on His mercy, commit themselves to His protection. The cause or ground of this hope or trust is also expressed: God mercifully cherishes His own people, whom He has once graciously received into His favor.