15 “We have passed through fire and water.” Fire and water are both dangerous in this life. Certainly water seems to extinguish fire, and fire seems to dry up water. Thus also these are the trials, wherein abounds this life. Fire burns, water corrupts: both must be feared, both the burning of tribulation and the water of corruption. Whenever there is adversity, and anything which is called unhappiness in this world, there is as it were fire: whenever there is prosperity, and the world's plenty flows about one, there is as it were water.
See that fire burn you not, nor water corrupt....Hasten not to the water: through fire pass over to the water, that you may pass over the water also. Therefore also in the mystic rites and in catechising and in exorcising, there is first used fire. For whence ofttimes do the unclean spirits cry out, “I burn,” if that is not fire? But after the fire of Exorcism we come to Baptism: so that from fire to water, from water unto refreshment. But as in the Sacraments, so it is in the temptations of this world: the straitness of fear draws near first, in place of fire; afterwards fear being removed, we ought to be afraid lest worldly happiness corrupt.
But when the fire has not made you burst, and when you have not sunk in the water, but hast swum out; through discipline you pass over to rest, and passing over through fire and water, you are led forth into a place of refreshment. For of those things whereof the signs are in the Sacraments, there are the very realities in that perfection of life everlasting....But we are not torpid there, but we rest: nor though it be called heat, shall we be hot there, but we shall be fervent in spirit. Observe that same heat in another Psalm: “nor is there any one that hides himself from the heat thereof.” What says also the Apostle? “In spirit fervent.” Therefore, “we have gone over through fire and water: and You have led us forth into a cool place.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)