Know ye that our brother Timothy - The word ἡμων, our, which is supplied by our translators, is very probably genuine, as it is found in ACD*, ten others, the Syriac, Erpen's Arabic, the Coptic, Armenian, Slavonic, and Vulgate.
Is set at liberty - Απολελυμενον· Is sent away; for there is no evidence that Timothy had been imprisoned. It is probable that the apostle refers here to his being sent into Macedonia, Philemon 2:19-24, in order that he might bring the apostle an account of the affairs of the Church in that country. In none of St. Paul's epistles, written during his confinement in Rome, does he give any intimation of Timothy's imprisonment, although it appears from Philemon 1:1; Colossians 1:1; Philemon 1:1; that he was with Paul during the greatest part of the time.
With whom, if he come shortly, I will see you - Therefore Paul himself, or the writer of this epistle, was now at liberty, as he had the disposal of his person and time in his own power. Some suppose that Timothy did actually visit Paul about this time, and that both together visited the Churches in Judea.
Know ye that our brother Timothy is set at liberty - Or, is sent away. So it is rendered by Prof. Stuart and others. On the meaning of this, and its importance in determining who was the author of the Epistle, see the Introduction section 3, (5) 4, and Prof. Stuart‘s Introduction, section 19. This is a strong circumstance showing that Paul was the author of the Epistle, for from the first acquaintance of Timothy with Paul he is represented as his constant companion, and spoken of as a brother; 2 Corinthians 1:1 note; Philemon 1:1 note; Colossians 1:1 note; Philemon 2:19, Philemon 2:23, Paul, then a prisoner at Rome, speaks of the hope which he entertained that he would be able to send Timothy to them as soon as he should know how it would go with him. He designed to retain him until that point was settled, as his presence with him would be important until then, and then to send him to give consolation to the Philippians, and to look into the condition of the church. Now the passage before us agrees well with the supposition that that event had occurred - that Paul had ascertained with sufficient clearness that he would be released, so that he might be permitted yet to visit the Hebrew Christians, that he had sent Timothy to Philippi and was waiting for his return; that as soon as he should return he would be prepared to visit them; and that in the mean time while Timothy was absent, he wrote to them this Epistle.
(3) the supposition agrees well with the meaning of the word used here - ἀπολύω apoluōIt denotes properly, to let loose from: to loosen; to unbind; to release; to let go free; to put away or divorce; to dismiss simply, or let go, or send away; see Matthew 14:15, Matthew 14:22-23; Matthew 15:32, Matthew 15:39; Luke 9:12, et al.; compare Robinson‘s Lexicon and Stuart‘s Introduction, section 19. The meaning, then, I take to be this, that Timothy was then sent away on some important embassage; that the apostle expected his speedy return; and that then he trusted that he would be able with him to visit those to whom this Epistle was written.