John Calvin Commentary Isaiah 43:2

"When thou passest through the waters, I will be with thee; and through the rivers, they shall not overflow thee: when thou walkest through the fire, thou shalt not be burned, neither shall the flame kindle upon thee." — Isaiah 43:2 (ASV)

When thou shalt pass through the waters. This is an anticipation by which he declares that those who rely on God’s immediate assistance have no reason for sinking under adversity. That is stated more fully than in the preceding verse, because while he shows that the Church will not be exempt from calamities and afflictions, but must maintain a constant warfare, he encourages patience and courage; as if he had said, “The Lord has not redeemed you that you might enjoy pleasures and luxuries, or that you might abandon yourself to ease and indolence, but rather that you should be prepared for enduring every kind of evil.”

By fire and water he means every kind of misery to which we are liable in this life, for we must contend not with calamities of one kind only, but with infinitely diversified calamities. At one time we must pass through waters and at another through fire (Psalms 66:12). In like manner, the Apostle James exhorts believers not to faint when they fall into various temptations (James 1:2).

Indeed, faith needs to be put to the trial in many ways, for it often happens that he who has been victorious in one combat is baffled by another kind of temptation. We are therefore tried by afflictions, but are eventually delivered; we are baffled by the billows, but are not swallowed up; we are even scorched by the flames, but are not consumed. We have, indeed, the same feeling of pain as other people, but we are supported by the grace of God, and fortified by the spirit of patience, that we may not faint; and eventually he will stretch out his hand and lift us up on high.