First, if Paul believed and taught that with Jesus and his death and resurrection something had happened, a one-off event through which the world was now irrevocably different, so he also believed that, when he announced the message about Jesus (the “good news,” the “gospel”), a similar one-off event could and would take place in the hearts, minds, and lives of some of his hearers. Paul speaks about this one-off event with the term “power”: the power of the gospel, the power of the spirit in and through the gospel, or the power of “the word of God.”
