"Who do you say I am?" Jesus asked his disciples. Peter answered. "You're the Christ, the Son of the living God." Jesus said the Spirit of God revealed it to him.
When Jesus mentions his death, burial, and resurrection, Peter's response is different. "Be it far from you, Lord; this shall not be unto you."
"Get thee behind me, Satan," Jesus said.
What changed? Peter is still saved, but Satan used him. Satan also used him when he cut off the soldier's ear during Christ's arrest. And he used him when Peter denied Christ three times. The Book of Acts tells us that God used him in a mighty way after this.
Who is using you? If you act ugly, hateful, and sinful, it's Satan. If you are led by the Spirit, God is using you.
Romans 6:22 But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life.
They will know you by your fruits. Who is using you?