9 Then therefore, when matters were in this state, some of the enemies of Paul, desiring to fan up the war to greater vehemence, and to make the hatred of the tyrant, which was felt towards him greater, pretended that they themselves also preached; (and they did preach the right and sound faith,) for the sake of the doctrine advancing more rapidly: and this they did, not with the desire to disseminate the faith; but in order that Nero, having learned that the preaching was increasing and the doctrine advancing, might the sooner have Paul led away to execution. There were therefore two schools; that of Paul's scholars and that of Paul's enemies; the one preaching out of sincerity, and the others out of love of contention and the hatred they felt towards Paul.
And by way of declaring this he said, “Some indeed through envy and strife are preaching Christ,” (pointing out those his enemies) “but some also through good pleasure;” saying this about his own scholars. Then next about those; “Some indeed out of contentiousness,” (his enemies,) not purely, not soundly, but, “thinking that they are thereby bringing pressure upon my bonds; but the others out of love;” (this again about his own brethren;) “knowing that I am set for the defence of the gospel.”
For what? Nevertheless, in any way; whether in pretence or in sincerity, Christ is being announced. So that vainly and to no purpose is this saying taken in reference to heresies. For those who then were preaching were not preaching corrupt doctrine; but sound and right belief. For if they were preaching corrupt doctrine, and were teaching other things contrary to Paul, what they desired was certain not to succeed to them. Now what did they desire? That the faith having grown, and the disciples of Paul having become numerous, it should rouse Nero to greater hostility.
And if they were preaching different doctrines, they would not have made the disciples of Paul numerous; and by not doing so, they would not have exasperated the tyrant. He does not therefore say this— that they were bringing in corrupt doctrines— but that the motive from which they were preaching, this was corrupt. For it is one thing to state the pretext of their preaching itself was not sound. For the preaching does not become sound when the doctrine is laden with deception; and the pretext does not become sound when the preaching indeed is sound, but they who preach do not preach for the sake of God, but either with a view of enmity, or with a view to the favour of others.
Source: Homily Concerning "Lowliness of Mind" (New Advent)