John Calvin Commentary Isaiah 26:4

"Trust ye in Jehovah for ever; for in Jehovah, [even] Jehovah, is an everlasting rock." — Isaiah 26:4 (ASV)

Trust ye in Jehovah for ever. Regarding the words, some read the second clause as, “Trust in God, the strong Jehovah of ages.” However, since צור (tzūr) is not always an adjective but signifies strength, I reject that meaning as forced; besides, it has little relation to the subject, as will immediately appear. There is also little basis for the ingenuity of those who infer Christ's divinity from this passage, as if the Prophet said that “Jehovah is in Jah,” for the twofold name of God is given for the express purpose of magnifying his power.

He now exhorts the people to rest safely on God; therefore, after the preceding doctrine, there is now room for exhortation. Besides, it would have been vain to say that our peace is in God's hand and that he is our faithful guardian, if we had not been taught and instructed on this subject and, at the same time, urged by exhortations.

Yet he exhorts us not only to earnest hope but also to perseverance. This discourse applies properly to believers, who have already learned what it is to trust in the Lord and who need to be strengthened, because they are still weak and may often fall because of the various motives for distrust with which they are called to struggle. He therefore does not enjoin them merely to trust in the Lord, but to remain steadfastly in trust and confidence to the end.

For in Jah Jehovah is the strength of ages. We ought to attend to the reason given here: namely, that just as God's power—the object of faith—is perpetual, so faith ought to be correspondingly perpetual. When the Prophet speaks of God's strength and power, he does not mean dormant power, but active and energetic power that is actually exerted on us and brings to completion what he began.

This doctrine has a wider application, for it directs us to truly believe that we ought to contemplate God's nature. As soon as we turn away from beholding it, we see nothing but what is fleeting, and then we immediately lose heart. Thus, faith ought to rise above the world by continual advances, for neither God's truth, nor his justice, nor his goodness is temporary or fading; instead, God always remains the same.