Dante’s process of entering the gates of Purgatory is symbolically a process of sacramental penance. At the shores of Purgatory we remember that Dante had humbled himself by being girt with a reed (Purg. I.133-36). Now at the gates of Purgatory, having humbled himself, Dante must confess his sins, bring his heart to contrition, and then he will be forgiven. The gates of Purgatory allegorically represent the liturgical process of the sacrament of penance that Dante must here undergo as the beginning of his journey up the mountain to fully purge those sins.
We then came on, and the first step was white marble so polished and clear that I mirrored myself in it in my true likeness; the second was darker than perse and was of a stone rugged and burnt, cracked in its length and in its breadth. The third, which lies massy above, seemed to me of porphyry as flaming red as blood that spurts from a vein (Purg. IX.94-102).
These three steps leading up to the gates of Purgatory represent the three steps of the sacrament of penance. The first marble step polished so clear that Dante can see his true likeness reflected in it corresponds to repentance and confession. During confession, the penitent examines his heart as in a mirror and lays bear all his sins. The second step corresponds to true contrition, by which the stubborn will of the sin-stained heart is broken. The final step at the gates corresponds to the atonement for our sins we find in Christ’s blood. It is here that forgiveness takes place.
However, this process is but the beginning. The repentant, contrite heart finds forgiveness in Christ’s blood, and while forgiveness is instant, he who is forgiven does not necessarily “feel” forgiven instantly (cf. C. S. Lewis in Screwtape Letters, 4). The broken heart must be mended in the right way; the wax must be reshaped; the loves, redirected. This is made clear as Dante receives the seven P’s upon his forehead representing the seven deadly sins of which he must be purged throughout the seven terraces of Purgatory. And as Dante says: ma tre volte nel petto pria mi diedi (but first I smote three times upon my breast, ln. 111). This corresponds to the liturgy: mea culpa, mea culpa, mea maxima culpa, in which the penitent confesses sins of thought, word and deed.
Now that Dante has undergone penance and has had his sins made plain for all to see (and has been forgiven of them), the healing starts with the purgation of these sins from the soul, right here at the gates of Purgatory where Dante will begin his ascension up the seven terraces. But as the angelic guard warns Dante, “Enter; but I bid you know that he who looks back returns outside again” (ln. 131-32). Dante has recognized his sins, broken his stubborn heart and has been covered with Christ’s blood. He has confessed his sins and they have been made plain for all to see. He must not look back on them now, he must not go back to them. Instead, he must move onward and upward to purge these sins from his soul and reorder the desires of his sinful heart.