John Chrysostom

I suppose that they were not at all unfavorably disposed towards Timothy: wherefore he also put him forward. For "know ye," he says, "that our brother Timothy is set at liberty, with whom, if he come shortly, I will see you." "Set at liberty," he says; from whence? I suppose he had been cast into prison: or if not this, that he was sent away from Athens. For this also is mentioned in the Acts. …

Homily on Hebrews 34 ↗

Theophylact of Ohrid

They were not particularly hostile toward Timothy, which is why he placed him first. From where then was he released? Either he was in prison and was released, or he was sent away from Athens, since he was there (cf. Phil. 2:19). And this is sufficient to persuade them to abandon their hostility toward him, when he is ready to come with a disciple. Perhaps this indeed happened, and he traveled to Judea from Rome. For the first time he was released by Nero, as we also said in the introduction to this epistle.

Commentary on Hebrews ↗

Oecumenius

"You should know that our brother Timothy has been released." Rather, they did not have a grudge against Timothy, which is why Paul writes about him. What does it mean, "released"? Either that he was in prison and was released; or that he was being released from Athens, because he was there. "with whom I shall see you if he comes soon." Timothy was released; but if he comes to me soon, I will see you together with him, coming to you. For it was likely that he had been released, but had not yet gone to Paul.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

Then he recommends the one through whom he writes, saying, You should understand that our brother Timothy has been released, namely, from prison, where he was with the Apostle. Or released by me to preach and come to you, both because he had been circumcised (Ac. 16:3) and because, with whom I will see you, if he comes soon. In this he shows the love he had for them. He also shows this because, even though he did not visit them, he was suffering in Rome and was uncertain whether he would be set at liberty for a while.

Commentary on Hebrews ↗