When Jesus calls Peter "Satan," commentators explain He is not identifying Peter as the devil, but as an "adversary" (the literal meaning of "Satan"). Peter's well-intentioned desire to protect Jesus from suffering was acting as a stumbling block to God's divine plan. As Spurgeon notes, Jesus looked through Peter and saw Satan using the apostle as a mouthpiece. This shows how even a beloved friend can become an obstacle when they prioritize human comfort over God's will.