I find then a law - There is a law whose operation I experience whenever I attempt to do good. There have been various opinions about the meaning of the word “law” in this place. It is evident that it is used here in a sense somewhat unusual. But it retains the notion which commonly attaches to it of what binds, or controls. And though this to which he refers differs from a law, inasmuch as it is not imposed by a superior, which is the usual idea of a law, yet it has so far the sense of law that it binds, controls, influences, or is that to which he was subject. There can be no doubt that he refers here to his carnal and corrupt nature; to the evil propensities and dispositions which were leading him astray. His representing this as a law is in accordance with all that he says of it, that it is servitude, that he is in bondage to it, and that it impedes his efforts to be holy and pure. The meaning is this, “I find a habit, a propensity, an influence of corrupt passions and desires, which, when I would do right, impedes my progress, and prevents my accomplishing what I would.” Compare Galatians 5:17. Every Christian is as much acquainted with this as was the apostle Paul.
Do good - Do right. Be perfect.
Evil - Some corrupt desire, or improper feeling, or evil propensity.
Is present with me - Is near; is at hand. It starts up unbidden, and undesired. It is in the path, and never leaves us, but is always ready to impede our going, and to turn us from our good designs; compare Psalms 65:3, “Iniquities prevail against me.’ The sense is, that to do evil is agreeable to our strong natural inclinations and passions.