It's a bit strange, but I think the context makes a bit more sense.

As I understand it, one of Christ's strongest apostles approaches him and tells him not to go to Jerusalem because he is sure to die. That someone else should go, so that he may live. Christ rejects this and accepts the responsibility of likely dying for the salvation of all mankind. He rejects this by saying something closer to "Get behind me, deceiver." As if to say: "You are not acting like yourself. Get back in line, follow me and support me to my destiny; and do no try to dissuade me from my responsibility."

The phrase has taken on a mystical form as later interpretations see the apostle as being possessed by Satan as a last ditch effort to stop Christ from saving mankind. In this incarnation, Christ sees the efforts of Satan, and literally calls him out. In this way, Christ is identifying that the apostle's loss of faith in Jesus's destiny is the work of Satan to actually stop him.

I like this interpretation a bit more because it poses a very interesting psychological dynamic. The lord of darkness, true evil, an apparently insurmountable force, can simply be called out and rejected. That evil is a choice. Christ didn't use any real magical incantations, he just called Satan's bluff.

To a Christian, this can be seen as the actual power of Christ in Christians. In the face of the most powerful evil to ever exist, all you have to do is command it to fuck off. That simple choice is enough to defeat the most powerful machinations of the most evil things to ever exist.

Now sure, you can't just use it as a magical incantation to not get shot or something, but it exists to remind you that the path of evil and darkness can simply be avoided by rejecting it as soon as you see it. It is an incredibly empowering statement, that could be summarized as "fuck off, devil".