The gospel is about Christ’s person and work, as testified by the apostles, who are the eyewitnesses and authorised spokespeople of his person and work. The content of their testimony – their gospel – is that Christ is God, expressing his character in redemptive action. A person who entrusts him/herself to that gospel – and in that gospel, to that apostolically-proclaimed Christ, and in Christ, to God – will inevitably be motivated to spread that gospel. A church founded on that apostolic gospel will inevitably express its evangelical (gospel-founded) character in redemptive proclamation (evangelism). The apostolic gospel is autodunamis – self-powered, self-propelled.
Christ engages is redemptive action; we engage in redemptive proclamation. Christ’s death and resurrection are unique events in actually reconciling people to God. Our actions of love and service are not on the same level. They express the reconciliation that we have with God and each other; they provide a “plausibility framework” that may help convince people of the truth of the gospel; but they do not in themselves reconcile anyone to God. People are right with God only when they entrust themselves to the crucified and risen Christ, as proclaimed by the apostles. The redemptive efficacy of the apostolic gospel is unique to itself. (I tried to think of a fancy word beginning with “auto” to express this, but can’t yet. Anyone…?)