If we would follow Jesus we must take certain definite steps. The first step, which follows the call, cuts the disciple off from his previous existence. The call to follow at once produces a new situation. To stay in the old situation makes discipleship impossible.

Levi must leave the receipt of custom and Peter his nets in order to follow Jesus. One would have thought that nothing so drastic was necessary at such an early stage. Could not Jesus have initiated the publican into some new religious experience, and leave them as they were before? He could have done so, had he not been the incarnate Son of God. But since he is the Christ, he must make it clear from the start that his word is not an abstract doctrine, but the re-creation of the whole life of man. The only right and proper way is quite literally to go with Jesus. The call to follow implies that there is only one way of believing on Jesus Christ, and that is by leaving all and going with the incarnate Son of God.

The first step places the disciple in the situation where faith is possible. If he refuses to follow and stays behind, he does not learn how to believe. He who is called must go out of his situation in which he cannot believe, into the situation in which, first and foremost, faith in possible. But this step is not the first stage of a career. Its sole justification is that it brings the disciple into fellowship with Jesus which will be victorious. So long as Levi sits at the receipt of custom, and Peter at his nets, they could both pursue their trade honestly and dutifully, and they might both enjoy religious experiences, old and new. But if they want to believe in God, the only way is to follow his incarnate Son. 

From the book The Cost of Discipleship, by Dietrich Bonhoeffer, pp.19-20