Acts 13:2

It was while these men were serving that God redirected them. Many have observed that it is easier to direct a ship that is in motion than one that is standing still. Similarly God often uses His servants who are already serving Him as they have opportunity rather than those who are not serving Him but just sitting by idly waiting for direction. Notice also that the ministry of these men, while to the church, was primarily to the Lord (cf. Colossians 3:24). Fasting in this context undoubtedly involved going without food temporarily to give attention to spiritual matters of greater importance than eating.

"Pious Jews of the time fasted twice each week, and early Christians may have continued the custom." [Note: Kent, p. 108.]

The Holy Spirit probably revealed His "call" through one or more of these prophets (cf. Acts 8:29; Acts 10:19; Acts 13:4). How He did it was less important to Luke than that He did it (cf. Acts 13:4). God leads His people though a variety of means that His disciples who are walking with Him can identify as His leading. If Luke had revealed just how the Spirit gave this "missionary call," every missionary candidate that followed might expect exactly the same type of leading. One commentator speculated as follows.

". . . this would seem to suggest that at a service of divine worship one of the prophets was moved by the Spirit to propose the mission of Paul and Barnabas." [Note: Neil, p. 154. See George W. Murray, "Paul’s Corporate Evangelism in the Book of Acts," Bibliotheca Sacra 155:618 (April-June 1998):189-200.]