10 He therefore does not say this— that they were bringing in heresies; but that it was not from a right motive, nor through piety that they were preaching what they did preach. For it was not they might increase the gospel that they were doing this; but that they might wage war against him, and throw him into greater danger— on this account he accuses them. And see how with exactitude he laid it. “Thinking,” he says, “that they were putting pressure upon my bonds.” He did not say, putting, but “thinking they were putting upon,” that is supposing, by way of pointing out that even if they so supposed, still he himself was not in such a position; but that he even rejoiced on account of the advance of the preaching.
He added therefore saying, “But in this I both rejoice and will rejoice:” whereas if he held their doctrines deception, and they were bringing in heresies, Paul could not possibly rejoice. But since the doctrine was sound and of genuine parentage, on this account he says, “I rejoice and will rejoice.” For what if they are destroying themselves by doing this out of contentiousness? Still, even unwillingly, they are strengthening my cause. Do you see how great is Paul's power?
How he is caught by no one of the devil's machinations? And not only is he not caught; but also by these themselves he subdues him. For great indeed is both the devil's craftiness, and the wickedness of those who minister to him; for under pretence of being of the same mind, they desired to extinguish the proclamation. But “he who seizes the cunning in their craftiness” did not permit that this should take place then. By way of declaring this very thing at least Paul said, “But the continuing in the flesh is the more necessary for your sake; and this I confidently know, that I shall continue and remain in company with you all.” For those men indeed set their mind on casting me out of the present life, and are ready to endure anything for this object: but God does not permit it on your account.
Source: Homily Concerning "Lowliness of Mind" (New Advent)