Explanation of the parable of the Prodigal Son, in which St. Ambrose applies it to refute the teaching of the Novatians, proving that reconciliation ought not to be refused to the greatest offender upon suitable proof of repentance.
13. And the Apostle does not contradict the plain teaching of Christ, Who set forth, as a comparison of a repentant sinner, one going to a foreign country after receiving all his substance from his father, wasted it in riotous living, and later, when feeding upon husks, longed for his father's bread and then gained the robe, the ring, the shoes, and the slaying of the calf, which is a likeness of the Passion of the Lord, whereby we receive forgiveness.
14. Well is it said that he went into a foreign country who is cut off from the sacred altar, for this is to be separated from that Jerusalem which is in heaven, from the citizenship and home of the saints. For which reason the Apostle says: “Therefore now you are no more strangers and foreigners, but fellow citizens with the saints and of the household of God.”
15. “And,” it is said, “wasted his substance.” Rightly, for he whose faith halts in bringing forth good works does consume it. For, “faith is the substance of things hoped for, the evidence of things not seen.” And faith is a good substance, the inheritance of our hope.
16. And no wonder if he was perishing for hunger, who lacked the divine nourishment, impelled by the want of which he says: “I will arise and go to my father, and will say unto him: Father, I have sinned against heaven, and before you.” Do you not see it plainly declared to us, that we are urged to prayer for the sake of gaining the sacrament? And do you wish to take away that for the sake of which penance is undertaken? Deprive the pilot of the hope of reaching port, and he will wander uncertainly here and there on the waves. Take away the crown from the athlete, and he will fail and lie on the course. Take from the fisher the power of catching his booty, and he will cease to cast the nets. How, then, can he, who suffers hunger in his soul, pray more earnestly to God, if he has no hope of the heavenly food?
17. “I have sinned,” he says, “against heaven, and before you.” He confesses what is clearly a sin unto death, that you may not think that any one doing penance is rightly shut out from pardon. For he who has sinned against heaven has sinned either against the kingdom of heaven, or against his own soul, which is a sin unto death, and against God, to Whom alone is said: “Against You only have I sinned, and done evil before You.”
18. So quickly does he gain forgiveness, that, as he is coming, and is still a great way off, his father meets him, gives him a kiss, which is the sign of sacred peace; orders the robe to be brought forth, which is the marriage garment, which if any one have not, he is shut out from the marriage feast; places the ring on his hand, which is the pledge of faith and the seal of the Holy Spirit; orders the shoes to be brought out, for he who is about to celebrate the Lord's Passover, about to feast on the Lamb, ought to have his feet protected against all attacks of spiritual wild beasts and the bite of the serpent; bids the calf to be slain, for “Christ our Passover has been sacrificed.” For as often as we receive the Blood of the Lord, we proclaim the death of the Lord. As, then, He was once slain for all, so whenever forgiveness of sins is granted, we receive the Sacrament of His Body, that through His Blood there may be remission of sins.
19. Therefore most evidently are we bidden by the teaching of the Lord to confer again the grace of the heavenly sacrament on those guilty even of the greatest sins, if they with open confession bear the penance due to their sin.
Source: On Repentance (New Advent)