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

It may be well for us to remark that we are not sufficiently grateful, I fear, for the preservation which God affords to us from fire…The walking through the fire here is put for the severest form of trouble: You have, in the commencement of the verse, trouble described as passing through the water. This represents the overwhelming influence of trial in which the soul is sometimes so covered that it becomes like a man sinking in the waves. “When thou goest through the rivers”—those mountain torrents which with terrific force are often sufficient to carry a man away. This expresses the force of trouble, the power with which it sometimes lifts a man from the foothold of his stability and carries him before it. “When thou passest through the rivers they shall not overflow thee.” But going through the fire expresses not so much the overwhelming character and the upsetting power of trouble as the actual consuming and destructive power of trouble and temptation. The metaphor is more vivid, more terrific, than that which is employed in the first sentence, and yet vivid and awful though it be, it is certainly not too strong a figure to be used as the emblem of the tribulations, temptations and afflictions, through which the Church and people of God have been called to pass. We may apprise the richness of the promise in proportion to the astonishing character of the metaphor, and we ought to value the privilege which it confers in the precise ratio of the dreadful character of the danger against which it preserves us. “When thou walkest through the fire, thou shalt not be burned, neither shall the flame kindle upon thee.”

Fire! Fire! Fire! by C. H. Spurgeon