The earliest followers of Jesus would have come to this Pentecost with a clear understanding of its meaning in their Jewish religion. Pentecost was the Greek name for Shavuot, the Feast of Weeks, celebrated seven weeks after Passover. Though originally a harvest festival, by the time of Jesus it had come to be a commemoration of God’s foundational covenant with Israel on Mount Sinai (in the book of Exodos). On that feast, the Lord chose the assembled Hebrews to become his people and given them a task.
Now, if you obey me completely and keep my covenant, you will be my treasured possession among all peoples, though all the earth is mine. You will be to me a kingdom of priests, a holy nation. That is what you must tell the Israelites. So Moses went and summoned the elders of the people. When he set before them all that the Lord had ordered him to tell them, all the people answered together, “Everything the Lord has said, we will do.”
When Jesus’s disciples were “all in one place together,” the Sinai foundational event was repeated: “The Lord appeared to them in tongues of fire. At this sound, they gathered in a large crowd, but they were confused because each one heard them speaking in his own language.” (Acts 2:6)
The Gospel of John gives its own version of the foundation and commission, “[Jesus] said to them again, ‘Peace be with you. As the Father has sent me, so I send you.’ And when he had said this, he breathed on them and said to them, ‘Receive the holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained.’” (Jn. 20:21-23) Though forgiveness and retention of sins relates in the first instance to admission or non-admission of outsiders to the community in a missionary sense, in a deeper sense it celebrates and illustrates the reconciliation won by the death of the Lamb (cf. 1 John 1:6-10) and in that sense it provides a Scriptural foundation for the Catholic tradition of sacramental forgiveness.