"As the Father has sent Me, I also send you..."
Jesus, speaking to His Apostles, tells them that as the Father sent Him, He also sends them. And the very next line is, "Receive the Holy Ghost... whose sins YOU forgive, they ARE forgiven them..." THIS is why His Apostles, our first bishops, have the authority to forgive sins AND since Jesus sent them to forgive sins - they too must pass on this authority to their successors - our bishops are those successors! THIS is why our priests, IF they have THIS authority from a valid bishop in succession from the Apostles, can forgive sins at the Sacrament of Reconciliation (aka, the Sacrament of Penance). Those sins THEY do not forgive (are retained) are NOT forgiven ("they are retained"). One who is not of this valid succession has NO AUTHORITY to forgive sins.
THIS is His Divine Mercy, which is what this Sunday is also referred to in the modern lectionary.
The Sunday After Easter is traditionally Low Sunday (Extraordinary Rite).
Some highlights from the Extraordinary Rite:
Introit: 1 Peter 2:2 - "As newborn babies, alleluia, desire the rational mile without guile, alleluia!"
Jesus saith to him: Because thou hast seen me, Thomas, thou hast believed: blessed are they that have not seen, and have believed. Many other signs also did Jesus in the sight of his disciples, which are not written in this book. But these are written, that you may believe that Jesus is the Christ, the Son of God: and that believing, you may have life in his name."