From my notes on his book The Callings: The Gospel in the World:
The call to the ministry is extraordinary, not in the sense that it is miraculous or accompanied by visions, but because “by it a man is taken out of many of the routine commitments of daily life.” Specifically, “he ought to be freed from the need to earn his daily living in order to give himself exclusively to the word of God (1 Tim 5:17).”
It also is extraordinary in that it arises out of the ordinary. A person generally will carry on a normal calling, and “it is when he is inwardly constrained to preach the gospel, and his gifts — his ability to handle Scripture, to preach, to give leadership — are recognized by the church, that his inward call becomes outwardly ratified. It is as these inward and outward circumstances combined that a man has a warrant for leaving his regular calling and attempting to obtain a position of pastoral oversight.”