Thomas Aquinas — on 2Tim 3:5 (Commentary on 2 Timothy)

“Then he mentions simulation, saying, having an appearance indeed of godliness: for such false apostles are deceitful women (2 Cor 11:13); but denying the power thereof, namely, the power of godliness, which is described in two ways here; hence he says, denying, i.e., not possessing the virtue: they profess to know God, but in their works deny him (Titus 1:16). In another way, because the power of a thing is that on which the whole thing depends. But the entire virtue of godliness depends on charity; therefore, he says, denying the power thereof, i.e., of charity. Above, the Apostle described the dangers to come in the last days, and the causes from which they shall arise; here he teaches that even in our own days they must be avoided: first, he advises him to avoid them; second, he points out certain contemporaries who personify these dangers, at for of these sort. He says, therefore: I have warned you that in the last days will appear men of the worst sort; but do not suppose that you are safe today, because even now you must avoid such persons, lest you slip into a like error: a man that is a heretic, after the first and second admonition, avoid (Titus 3:10). And although you must avoid them under some aspects, you must not neglect to persuade them.”