Symbolism, Imagery, Allegory
The three steps at the foot of the gate to Purgatory proper are an allegory for the Sacrament of Penance. They represent recognition of and contrition for one’s sins. The first stage in the Sacrament is contrition of the heart, represented by a step polished a brilliant “white… so clear that [Dante] was mirrored there.”
The mirroring reflects to the soul the nature of its sin so that it can feel shame. The second stage is confession of the lips, symbolized by a step whose “rough-textured,” cracked and “crumbling” appearance corresponds to the emotional turmoil a soul should feel upon confession, as if broken by the sins the person has committed. The third and final stage is satisfaction by works, represented by a “flaming red” step whose color is compared to spurting “blood.” This symbolizes the sweat and blood the penitent should shed in laboring to redeem himself, and recalls the blood Jesus shed on the cross when he made the ultimate sacrifice for mankind.
It's appropriate that these steps rise just outside Purgatory proper, for the penitents must recognize their guilt and shame before embarking on the “works” hinted at by the final step. Their labors in Purgatory proper will complete their penance.
The guardian angel who stands before the gate to Purgatory holds a sword. Traditionally the sword is the sign of a judge; here, it indicates the ability of this angel to pass judgment on the worthiness of the soul in question to enter Purgatory.
The angel also holds two keys. The silver key, which “needs much art and skill before it will unlock,” represents the complex art of judging and passing sentence on a sinner. Indeed, the name of the keys reflects this importance; they are the “keys of the kingdom of heaven.”