The Gospel at Mass was a passage from the Gospel of Mark in which Saint Peter does not fare well. In his lively homily, Cardinal DiNardo spoke of how such blunt descriptions are a defining characteristic of Mark's Gospel.
From Mark Chapter 8:
Jesus and his disciples set out for the villages of Caesarea Philippi.
Along the way he asked his disciples, “Who do people say that I am?”
They said in reply, “John the Baptist, others Elijah, still others one of the prophets.”
And he asked them, “But who do you say that I am?”
Peter said to him in reply, “You are the Christ.”
Then he warned them not to tell anyone about him.
He began to teach them that the Son of Man must suffer greatly and be rejected by the elders, the chief priests, and the scribes, and be killed, and rise after three days. He spoke this openly.
Then Peter took him aside and began to rebuke him.
At this he turned around and, looking at his disciples, rebuked Peter and said, “Get behind me, Satan. You are thinking not as God does, but as human beings do.”
He summoned the crowd with his disciples and said to them, “Whoever wishes to come after me must deny himself, take up his cross, and follow me. For whoever wishes to save his life will lose it, but whoever loses his life for my sake and that of the gospel will save it.”
For a homily on this passage, check out A Concord Pastor. Brother Patrick has a reflection, too.
The image of Saint Peter above is from here.