John Calvin Commentary Isaiah 55:11

"so shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it." — Isaiah 55:11 (ASV)

So shall my word be. The word goes out of the mouth of God in such a manner that it likewise “goes out of the mouth” of men; for God does not speak openly from heaven, but employs men as His instruments, so that through their agency He may make known His will.

But the authority of the promises is more fully confirmed when we are told that they proceed from the sacred mouth of God. Although He brings forward witnesses from the earth, He therefore declares that all that they have promised shall be ratified and sure.

And, in order to impress more deeply on the minds of men the power and efficacy of preaching, He declares that He does not cast that precious seed at random but appoints it for a fixed purpose. Consequently, we should entertain no doubt as to the effect.

For there is nothing to which mortals are more prone than to judge God by themselves and so withhold belief from His voice.

This doctrine must be frequently repeated and inculcated, that we may know that God will do what He has once spoken. For this reason, when we hear the promises of God, we should consider what His design in them is; so that, when He promises the free pardon of our sins, we may be fully assured that we are reconciled through Christ. But, as the word of God is efficacious for the salvation of believers, so it is abundantly efficacious for condemning the wicked, as Christ also teaches, The word which I have spoken, that shall judge him at the last day.