in biblical times. when jesus rebukes peter he is saying that peter at that moment is embodying the complete opposite of what jesus was trying to accomplish, as would satan.
21 From that time on, Jesus began to show his disciples that he must go to Jerusalem and undergo great suffering at the hands of the elders and chief priests and scribes, and be killed, and on the third day be raised. 22And Peter took him aside and began to rebuke him, saying, ‘God forbid it, Lord! This must never happen to you.’ 23But he turned and said to Peter, ‘Get behind me, Satan! You are a stumbling-block to me; for you are setting your mind not on divine things but on human things.’
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http://www.ncccusa.org/welcome/sitemap.html#NRSVobviously at that time satan was consider a real entity, in combat with the god of Israel. for those people who postulate the existence of god i think it would be reasonable that they do the same of god's opposite, satan. that would apply for theology today. if theology wants to talk about god, i think it must also talk about satan. if someone believes god is a real entity then believing satan is a real entity makes sense to me. most of what we read above about jesus suffering and rising on the third day is not jesus speaking but the evangelist. it is the early church retelling the resurrection experience. although, jesus yelling at peter is, according to some NT scholars, probably a real event passed down.