20 Therefore, brethren, let us be careful of our bodies, nor misuse them as though not our own. Let us not say like the heretics, that this vesture of the body belongs not to us, but let us be careful of it as our own; for we must give account to the Lord of all things done through the body. Say not, none sees me; think not, that there is no witness of the deed. Human witness oftentimes there is not; but He who fashioned us, an unerring witness, abides faithful in heaven, and beholds what you do.
And the stains of sin also remain in the body; for as when a wound has gone deep into the body, even if there has been a healing, the scar remains, so sin wounds soul and body, and the marks of its scars remain in all; and they are removed only from those who receive the washing of Baptism. The past wounds therefore of soul and body God heals by Baptism; against future ones let us one and all jointly guard ourselves, that we may keep this vestment of the body pure, and may not for practising fornication and sensual indulgence or any other sin for a short season, lose the salvation of heaven, but may inherit the eternal kingdom of God; of which may God, of His own grace, deem all of you worthy.
Source: Catechetical Lectures (New Advent)