The life of discipleship can only be maintained so long as nothing is allowed to become Christ and ourselves–neither the law, nor personal piety, nor even the world. This disciple always looks only to his master, never to Christ and the law, Christ and religion, Christ and the world. He avoids such notions like the plague. Only by following Christ alone can he preserve a single eye. His eye rests wholly on the light that comes from Christ, and has no darkness or ambiguity in it.
As the eye must be single, clear, and pure in order to keep light in the body, as hand and foot can receive light from no other source save the eye, as the foot stumbles and the hand misses its mark when the eye is dim, as the whole body is in darkness when the eye is blind: so the follower of Christ is in the light only so long as he looks simply to Christ and at nothing else in the world.
Dietrich Bonhoeffer, The Cost of Discipleship