“You are thinking not as God does, but as human beings do.”
We are naturally averse to suffering and difficulty; we naturally prioritize our wants and needs over others. When Jesus speaks to His disciples about the necessity of His Passion and Death, Peter naturally seeks to encourage Him like any of us would encourage a friend who was convinced of some inevitable doom. But just when Peter had confessed his faith in Christ as the Son of God—something revealed to him not by flesh and blood but by the Father (Matthew 16:16-17)—he falls back to thinking as a mere man.
Naturally.
When Jesus calls Peter “Satan” He is not insulting His dearest friend, but rather warning him about his way of thinking: Satan is faithless, but a follower of Christ must, as St. Paul writes, be one who walks “…by faith, not by sight,” (2 Corinthians 5:7). The Enemy often seeks to convince us that what we see, what we feel, what we experience here and now is all there is. But the Cross teaches us that while we might see only the Death of Christ, what we do not see is the Resurrection to come. We see the struggles of being faithful to Christ and His Church, but we do not always understand, or see what benefit it all does for our soul. We must have faith: we must trust in the One who sees it all, who teaches us how to deny ourselves and take up the Cross.
We are called to live supernaturally, to trust in the God we cannot see, to deny ourselves in order to more fully embrace Jesus: to forsake our own life that we might receive the life He poured out upon the Cross.