Patristic Wisdom: Looking to the Second Sunday of Easter

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, Jesus came and stood in the midst, and said to them, “Peace be with you.” When He had said this, He showed them His hands and His side. Then the disciples were glad when they saw the Lord. So Jesus said to them again, “Peace to you! As the Father has sent Me, I also send you.” And when He had said this, He breathed on them, and said to them, “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained.” (John 20:19–31)

These words, Receive the Holy Spirit, were said in place of, “you will receive,” in the upper room. Therefore the gift, conferred through breathing, pertains only to the power of binding and absolving, as will be seen. This is the fulfillment of what was said to Peter, “I will give you the keys of the kingdom.” It was necessary that He granted these gifts so that they might know that He was of the same nature and substance of the Spirit who would give them an overabundance of gifts. This is why He said, “He will take what is mine and declare it to you.” Consider, however, how far this breathing extends, If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained. What truly wonderful gifts! Indeed [the Spirit] not only gives power over the elements and the ability to perform signs and wonders, but even concedes that God has called them by name, and that which is only appropriate to Himself is given to His servants. The prerogative to absolve and retain sins only belongs to God. The Jews sometimes raised this objection to the Savior, saying, “Who can forgive sins but God alone?” And yet the Lord generously gave this power to those who feared Him.

Theodore of Mopsuestia, Commentary on the Gospel of John 7.20.25