Continued reflection on the gospel must inevitably come to the question of the power of the gospel. What happens when the gospel is preached?
Peter's Pentecost sermon in Acts 2 is instructive here. After explaining that the Spirit of God had just been poured out as the fulfillment of Joel 2, Peter went on to provide an extended defense of Jesus' resurrection. He proclaimed that despite the Jews' betrayal of Jesus to the Romans and his subsequent execution at their hands, God raised Jesus up freeing him from death (22-24). He then argued that David was speaking of Jesus when he said prophesied, "For you will not abandon my soul to Hades or let your Holy One experience corruption" (27). Peter then went on to announce Jesus' sovereign lordship witnessing to his resurrection and saying that Jesus had been exalted to the right hand of God (33). Peter concluded his sermon saying, "Therefore let the entire house of Israel know with certainty that God has made him both Lord and Messiah, this Jesus whom you crucified" (36).
In this first post-Pentecost and Spirit-filled sermon, Peter preached a gospel highly consistent with what he have seen to be Paul's basic gospel announcement. Both focus on the death and resurrection of Jesus and his exaltation to a place of universal Lordship over all. The interesting thing is what happens next. Acts 2:37 indicates that when those around Peter heard him, "they were cut to the heart and said to Peter and to the other apostles, 'Brothers, what should we do?'" Upon hearing the good news that Jesus is Lord and God raised him from the dead, these persons began to experience conviction over their sin, perhaps specifically here in betraying Jesus to the Romans.
The point is that the Spirit of God uses the gospel as a means of grace to convict of sin and enable a response of faith to the news of Jesus' lordship and resurrection. Peter then commanded them to repent and be baptized in the name of Jesus Christ for the forgiveness of sin. Then he said, "You will receive the gift of the Holy Spirit" (38). It is important to note that the Spirit of God began to work graciously to convict prior to the actual forgiveness of sins and indwelling of the Spirit. This is an description of God's enabling grace that comes before a the experience of new life in him.
So, what is the power of the gospel? How does it work? When the gospel of Jesus' Lordship and resurrection is preached or the story is told, then the Spirit of God goes to work to convict of sin and graciously enable a response of faith. The gospel is God's chosen instrument to initiate the reconciliation of fallen human beings to himself. As Paul says in Romans 1:16, "[The gospel] is the power of God for salvation."