John’s gospel was written a couple of generations after the resurrection of Jesus. On the political scene, the Temple in Jerusalem had been destroyed by the Roman army a generation earlier than John's gospel. On the religious scene, the early Christians were considered a threat to Judaism because they wouldn’t observe the ritual things (such as ritual circumcision and various purity/dietary laws), not to mention their assertion that Jesus was indeed the Messiah. So the Christians were banned from the Synagogues. This led to a vigorous argument between the two groups, and it shows in several places in John’s gospel. Closer to the time of Jesus’ passion, we saw how Saul (Paul) and other Pharisees persecuted the Christians, with Saul even following them into Syria to arrest them and bring them back to Jerusalem for punishment. Even Deacon (St) Stephen had been stoned to death for defending his faith.
All of these things were indicated or hinted at in the earlier conflicts with the Pharisees, and the arrest, trial and execution of Jesus the Christ. Peter showed his personal fear during that process, denying he even knew Jesus. For all of those reasons, the apostles had real cause to “fear the Judeans” (the actual Greek words). But when Jesus appeared, his Presence brought Peace.
Jesus gave the apostles his Peace, and the awesome gift of the power of sanctifying: “For those whose sins you forgive, they are forgiven.” In this way they became 'sacramental' signs of Christ. In turn, this power was entrusted by them to their bishop successors. For this reason, we trust their teachings.