23 Tell me not that the body is a cause of sin. For if the body is a cause of sin, why does not a dead body sin? Put a sword in the right hand of one just dead, and no murder takes place. Let beauties of every kind pass before a youth just dead, and no impure desire arises. Why? Because the body sins not of itself, but the soul through the body. The body is an instrument, and, as it were, a garment and robe of the soul: and if by this latter it be given over to fornication, it becomes defiled: but if it dwell with a holy soul, it becomes a temple of the Holy Ghost. It is not I that say this, but the Apostle Paul has said, Do you not know, that your bodies are the temple of the Holy Ghost which is in you? Be tender, therefore, of your body as being a temple of the Holy Ghost. Pollute not your flesh in fornication: defile not this your fairest robe: and if ever you have defiled it, now cleanse it by repentance: get yourself washed, while time permits.
Source: Catechetical Lectures (New Advent)