Christ among us, not an ideology

…the Christian is in a different position from other people who are trying to be good. They hope, by being good, to please God if there is one; or—if they think there is not—at least they hope to deserve approval from good men. But the Christian thinks any good he does comes from the Christ-life inside him. He does not think that God will love us because we are good, but that God will make us good because He loves us; just as the roof of a greenhouse does not attract the sun because it is bright, but becomes bright because the sun shines on it.

And let me make it quite clear that when Christians say the Christ-life is in them, they do not mean simply something mental or moral. When they speak of being "in Christ" or of Christ being "in them," this is not simply a way of saying that they are thinking about Christ or copying Him. They mean that Christ is actually operating through them; that the whole mass of Christians are the physical organism through which Christ acts—that we are His fingers and muscles, the cells of His body. And perhaps that explains one or two things. It explains why this new life is spread not only by purely mental acts like belief, but by bodily acts like baptism and Holy Communion. It is not merely the spreading of an idea; it is more like evolution—a biological or super-biological fact. There is no good trying to be more spiritual than God. God never meant man to be a purely spiritual creature. That is why He uses material things like bread and wine to put the new life into us. We may think this rather crude and unspiritual. God does not: He invented eating. He likes matter. He invented it.

— C. S. Lewis, Mere Christianity, Book 2, What Christians Believe, Chapter 5, The Practical Conclusion