Jamieson, Fausset and Brown on Acts 13:2

2. As they ministered to the Lord--The word denotes the performance of official duties of any kind, and was used to express the priestly functions under the Old Testament. Here it signifies the corresponding ministrations of the Christian Church.

and fasted--As this was done in other cases on special occasions (Ac 13:3, 14, 23), it is not improbable that they had been led to expect some such prophetic announcement at this time.

the Holy Ghost said--through some of the prophets mentioned in Ac 13:1.

Separate me--So Ro 1:1.

for the work whereunto I have called them--by some communication, perhaps, to themselves: in the case of Saul at least, such a designation was indicated from the first (Ac 22:21). Note.--While the personality of the Holy Ghost is manifest from this language, His supreme divinity will appear equally so by comparing it with He 5:4.