4 “Mary Magdalene came and told the disciples, I have seen the Lord, and He has spoken these things unto me. Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus, and stood in the midst, and says unto them, Peace be unto you. And when He had so said, He showed unto them His hands and His side.” For nails had pierced His hands, a spear had laid open His side: and there the marks of the wounds are preserved for healing the hearts of the doubting.
But the shutting of doors presented no obstacle to the matter of His body, wherein Godhead resided. He indeed could enter without their being opened, by whose birth the virginity of His mother remained inviolate, “Then were the disciples glad when they saw the Lord. Then said He unto them again, Peace be unto you.” Reiteration is confirmation; for He Himself gives by the prophet a promised peace upon peace. “As the Father has sent me,” He adds, “even so send I you.” We know the Son to be equal to the Father; but here we recognize the words of the Mediator.
For He exhibits Himself as occupying a middle position when He says, He me, and I you. “And when He had said this, He breathed on them, and said unto them, Receive the Holy Ghost.” By breathing on them He signified that the Holy Spirit was the Spirit, not of the Father alone, but likewise His own. “Whose soever sins,” He continues, “ye remit, they are remitted unto them; and whose soever ye retain, they are retained.” The Church's love, which is shed abroad in our hearts by the Holy Spirit, discharges the sins of all who are partakers with itself, but retains the sins of those who have no participation therein. Therefore it is, that after saying “Receive the Holy Ghost,” He straightway added this regarding the remission and retention of sins.
Source: Tractates on the Gospel of John (New Advent)