John Calvin Commentary Isaiah 51:15

"For I am Jehovah thy God, who stirreth up the sea, so that the waves thereof roar: Jehovah of hosts is his name." — Isaiah 51:15 (ASV)

And I am Jehovah your God. Again the Lord declares his power, for so great is the unbelief and sluggishness of men that, although it is frequently declared, yet the very smallest temptation shows that they are not fully convinced of it. They quickly fall back upon themselves when they are hard-pressed by afflictions; and when they hear that anything is in the power of God, they do not think that it belongs to them.

Who divides the sea. He does not speak in general terms, but brings forward the instance which he had often mentioned before; for, by once redeeming the fathers, he held out to posterity the hope of eternal salvation. Justly, therefore, does he exclaim that he is the same God who long ago divided the sea; and next he magnifies the miracle by saying that its roaring billows were stilled at his command (Exodus 14:21).

We should know, therefore, that there are no raging billows which God cannot subdue and calm in order to deliver his Church. It is he who, by his power, stills the sea and makes it calm (Job 26:12), though it rages furiously; and he likewise drives and swells its waves when he sees fit. Literally, as I have remarked, the Prophet alludes to the history of the deliverance from Egypt.

Jehovah of hosts. The Lord is adorned with this title, that we may know how extensive his power is; and he exhibits that power as often as he is pleased to render assistance to his Church.