Adam Clarke on Acts 21:11

Took Paul's girdle, and bound his own hands, etc. - This was no doubt a prophet, in the commonly received sense of the term; and his mode of acting was like that of the ancient prophets, who often accompanied their predictions with significant emblems. Jeremiah was commanded to bury his girdle by the river Euphrates, to mark out the captivity of the Jews. Jer 13:4. For more examples of this figurative or symbolical prophesying, see Jer 27:2, Jer 27:3; Jer 28:4; Isa 20:1-6; Ezekiel 4:1-17; 12:1-28, etc.

Into the hands of the Gentiles - That is, the Romans, for the Jews had not, properly speaking, the power of life and death. And, as Agabus said he should be delivered into the hands of the Gentiles, he showed thereby that they would attempt to destroy his life. This prediction of Agabus was literally fulfilled: see Act 21:33.