John Calvin Commentary Psalms 89:34

"My covenant will I not break, Nor alter the thing that is gone out of my lips." — Psalms 89:34 (ASV)

My covenant will I not break. Since the true knowledge of God’s mercy can only be obtained from His word, He urges us to keep our eyes intently focused on His covenant. The more excellent and invaluable a blessing it is, “Never to be rejected after having been once adopted by Him,” the more difficult it is for us to believe its truth.

And we know how many thoughts from time to time present themselves to our minds, tempting us to question it. So that the faithful, therefore, may not harass themselves excessively in debating in their own minds whether or not they are in favor with God, they are urged to look to the covenant and to embrace the salvation which is offered to them in it.

God here commends to us His own faithfulness, so that we may consider His promise sufficient and not seek the certainty of our salvation anywhere else. He had said above, If the children of David break my statutes; and now, alluding to that breach, He declares that He will not repay them as they repay Him: My covenant will I not break.

This implies that although His people may not fully act in a manner corresponding to their vocation, as they should, He will not allow His covenant to be broken and annulled because of their fault. He will promptly and effectually prevent this by blotting out their sins through a free pardon.

He is still pursuing the illustration of the previous proposition, I will not suffer my faithfulness to fail. He promises not only to be faithful on His part, as we say, but also that what He has promised will take full effect, despite all the obstacles that people may place in the way.

For He will strive against their sins, so that these sins do not prevent the fruit of His goodness from reaching them.

When the Jews, by their ingratitude and treachery, rebelled against Him, the covenant was not annulled, because it was founded upon the perfect immutability of His nature. And still today, when our sins reach even to the heavens, the goodness of God does not fail to rise above them, since it is far above the heavens.