What would you say is at the heart of Christian belief? There are certainly doctrinal beliefs that - in theory - should transform the way that a person sees the world but I'm not sure that those are truly the center of our faith. In fact, when we take the time to examine the teachings of Jesus and the teachings of the apostles to the early church, we begin to get another message: Christianity is about doing far more than it's about believing.
That said, action and belief are not opposed to one another. We act in certain ways because of the things we believe and, hopefully, others recognize our faith because of our actions. However, the epistle 1 John, from where we draw this Sunday's scripture reading, says that we should emphasize our actions and truth telling over declaring our faith. The writer of the epistle even goes so far as to say that the love of God isn't in someone's heart if they see a person in need and choose not to act.
Our faith is displayed in our actions and the two cannot be picked apart. We cannot simply say we believe and "talk the talk." We need to show our faith as we "walk the walk," living as incarnations of Christ in our world today and every day.