ESV - 23 But he turned and said to Peter, "Get behind me, Satan! You are a hindrance to me. For you are not setting your mind on the things of God, but on the things of man.
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The word Satan simply means "adversary." At that time Peter became an adversary to the Lord.
It helps me to remember that even today in 2020, more so than in the first century, evil and sin are so ingrained and deluded in our world and psyche and, if we are not gospel conscious, we will easily operate from the enemy's camp. Evil in the world today is like mixed red or blue cool-aid in water... it cannot be separated again. Only in Christ can we discern good and evil.