Jamieson, Fausset and Brown on Acts 21:11-14

11-14. So shall the Jews bind the man that owneth this girdle, &c.--For though the Romans did it, it was at the Jews' instigation (Ac 21:33; Ac 28:17). Such dramatic methods of announcing important future events would bring the old prophets to remembrance. (Compare Is 20:2, &c.; Jr 13:1, and Eze 5:1, &c.). This prediction and that at Tyre (Ac 21:4) were intended, not to prohibit him from going, but to put his courage to the test and when he stood the test, to deepen and mature it.