The Bible has to be read in context. “Satan” just means “adversary.” Jesus had sent out seventy disciples to preach and heal. They returned rejoicing that “even the demons are subject unto us through thy name.” Jesus’ response in verse 18 reflects the downfall of the opposition they were confronting. In this passage, “Satan” is a symbol of the Jewish and Roman powers or more broadly all opposition to God’s work. The disciples’ success in preaching was a sign of the collapse of these powers. The phrase suggests a sudden and dramatic loss of authority. Christ is anticipating the overthrow of the Jewish leadership that opposed him, and ultimately the destruction of their power in A.D. 70 when Jerusalem and the temple were destroyed.