As Augustine says (De Civ. Dei i, 13): "If a father's coat or ring, or anything else of that kind, is so much more cherished by his children, as love for one's parents is greater, in no way are the bodies themselves to be despised, which are much more intimately and closely united to us than any garment; for they belong to man's very nature." It is clear from this that he who has a certain affection for anyone, venerates whatever of his is left after his death, not only his body and the parts thereof, but even external things, such as his clothes, and such like. Now it is manifest that we should show honor to the saints of God, as being members of Christ, the children and friends of God, and our intercessors. Wherefore in memory of them we ought to honor any relics of theirs in a fitting manner: principally their bodies, which were temples, and organs of the Holy Ghost dwelling and operating in them, and are destined to be likened to the body of Christ by the glory of the Resurrection. Hence God Himself fittingly honors such relics by working miracles at their presence.

ST III, Q. 25, A. 6.

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