Acts 16:3

Him would Paul have to go forth with him--This is in harmony with all we read in the Acts and Epistles of Paul's affectionate and confiding disposition. He had no relative ties which were of service to him in his work; his companions were few and changing; and though Silas would supply the place of Barnabas, it was no weakness to yearn for the society of one who might become, what Mark once appeared to be, a son in the Gospel [HOWSON]. And such he indeed proved to be, the most attached and serviceable of his associates (Phi 2:19-23; Co1 4:17; Co1 16:10-11; Th1 3:1-6). His double connection, with the Jews by the mother's side and the Gentiles by the father's, would strike the apostle as a peculiar qualification for his own sphere of labor. "So far as appears, Timothy is the first Gentile who after his conversion comes before us as a regular missionary; for what is said of Titus (Gal 2:3) refers to a later period" [WIES]. But before his departure, Paul took and circumcised him--a rite which every Israelite might perform. because of the Jews . . . for they knew all that his father was a Greek--This seems to imply that the father was no proselyte. Against the wishes of a Gentile father no Jewish mother was, as the Jews themselves say, permitted to circumcise her son.

— Jamieson-Fausset-Brown Commentary (public domain)