John Calvin Commentary Psalms 106:23

"Therefore he said that he would destroy them, Had not Moses his chosen stood before him in the breach, To turn away his wrath, lest he should destroy [them]." — Psalms 106:23 (ASV)

And he said The prophet informs us, by these words, that the people had a keen sense of their remarkable deliverance from impending destruction, by means of prayer alone, which, for a time, restrained God’s vengeance from bursting forth against them. In a very short time, however, they returned to their customary state of mind, a striking proof of the terrible perversity of their hearts.

To represent how highly God was offended, the prophet says that he had purposed to destroy the transgressors. This is not because God is subject to human passions, to be very angry for a short time, and then immediately afterward, on being appeased, changes his purpose. For God, in his secret counsel, had resolved upon their forgiveness, just as he actually did pardon them.

But the prophet mentions another purpose, by which God designed to strike the people with terror, so that by coming to know and acknowledge the greatness of their sin, they might be humbled because of it. This is that repentance so frequently referred to in the Scriptures. It is not that God is changeable in himself; but he speaks after the manner of men, that we may be affected with a keener sense of his wrath. This is like a king who had resolved to pardon an offender, yet placed him before his judgment-seat, to impress him more effectively with the magnitude of the kindness done to him.

God, therefore, while he kept his secret purpose to himself, declared openly to the people that they had committed a trespass which deserved to be punished with eternal death. Next, the prophet says that Moses stood in the breach, meaning that he had made intercession with God, so that his terrible vengeance might not break forth among the people.

There is here an allusion to the way cities are stormed. For if a breach is made in the wall by any of the various engines employed in war, brave soldiers will instantly throw themselves into the breach to defend it. Therefore, Ezekiel reproaches the false prophets who, unlike Moses, deceived the people with their flatteries, making, so to speak, a mud-wall, and did not place themselves in the breach in the day of battle.

Ye have not gone up into the gaps, neither made up the hedge for the house of Israel, to stand in the battle in the day of the Lord, (Ezekiel 13:5).

Some commentators believe that the prophet refers to the separation which the people had made among themselves in violating the covenant of God, and the sacred relation in which they stood to each other; but the meaning is the same. For in that breach which gave rise to this metaphor or comparison, God, in defending his people so faithfully, was to them as a wall or bulwark. Having provoked him to anger again, he was about to rush upon them for their destruction, if Moses had not interposed as their intercessor.