There are two places in Scripture where the curious detail of a “charcoal fire” is mentioned.
One is in today's Gospel, where the Apostles return from fishing to find bread and fish warming on the fire.
The other is in the scene in the High Priest's courtyard on Holy Thursday, where Peter and some guards and slaves warm themselves while Jesus is being interrogated inside (see John 18:18).
At the first fire, Peter denied knowing Jesus three times, as Jesus had predicted (see John 13:38; 18:15-18, 25-27).
Today's charcoal fire becomes the scene of Peter's repentance, as three times Jesus asks him to make a profession of love. Jesus' thrice repeated command “feed My sheep” shows that Peter is being appointed as the shepherd of the Lord's entire flock, the head of His Church (see also Luke 22:32).
Jesus' question: “Do you love me more than these?” is a pointed reminder of Peter's pledge to lay down his life for Jesus, even if the other Apostles might weaken (see John 13:37; Matthew 26:33; Luke 22:33).
Jesus then explains just what Peter's love and leadership will require, foretelling Peter's death by crucifixion (“you will stretch out your hands”).
Before His own death, Jesus had warned the Apostles that they would be hated as He was hated, that they would suffer as He suffered (see Matthew 10:16-19,22; John 15:18-20; 16:2).
We see the beginnings of that persecution in today's First Reading. Flogged as Jesus was, the Apostles nonetheless leave “rejoicing that they have been found worthy to suffer.”
Their joy is based on their faith that God will change their “mourning into dancing,” as we sing in today's Psalm. By their sufferings, the know, they will be counted worthy to stand in heaven before “the Lamb that was slain,” a scene glimpsed in today's Second Reading (see also Revelation 6:9-11).