Dietrich Bonhoeffer:
Discipleship means adherence to Christ, and, because Christ is the object of that adherence, it must take the form of discipleship.
An abstract Christology, a doctrinal system, a general religious knowledge on the subject of grace or on the forgiveness of sins, render discipleship superfluous, and in fact they positively exclude any idea of discipleship whatever, and are essentially inimical to the whole conception of following Christ.
With an abstract idea it is possible to enter into a relation of formal knowledge, to become enthusiastic about it, and perhaps even to put it into practice; but it can never be followed in personal obedience.
Christianity without the living Christ is inevitably Christianity without discipleship, and Christianity without discipleship is always Christianity without Christ.
The Cost of Discipleship, 1937, (New York: Touchstone, 1995), 59, paragraphing added.