Observe, 2. What is here asserted concerning him that is born of God, namely, that he doth not commit sin, and that he cannot sin.
1. He doth not commit sin, he is no evil doer, no worker of iniquity, no habitual or customary sinner; he goes not on in any way or course of sin, as the wicked does, who makes a trade of it; yea, he doth not tolerate or allow himself in any single act of sin; not that he is absolutely free from all sin.
2. It is said, he cannot sin; but how? And why? How can he not sin? Has he not a natural power to sin? Has he not corruptions within, and temptations without, inclining and disposing him to sin, as well as other men? And has he not opportunity to sin; the same expectations of advantages by sin with other men? Yes, no doubt; but he has not a will bent for sin, or a heart and mind set upon sin, as the wicked have; nay, he has a heart and will opposite to sin, and set against all sin.
A gracious person then, though he hath not a natural impossibility, yet he has a moral impossibility to sin. He that is born of God hath a power to do that evil which he hath not a will to do; he hath always a natural power, and sometimes a civil power, as being in authority; but his blood and pedigree are so high, being born of God, that he disdains to meddle with, or to trade in so base a thing as sin is.
Note, That a child of God has a blessed impotency in the unregenerate part, that he cannot sin strongly, though as yet he has not that ability in the regenerate part, as not to sin at all.
Observe, 3. The reasons assigned why a regenerate person cannot sin as the wicked sin, because his seed remaineth in him, and because he is born of God; that is, he has an inward principle inclining and disposing him to hate and oppose all sin, to wit, the sanctifying grace of God; and he has that mortifying Spirit, which causes him daily more and more to die unto sin, and enables him to mortify the deeds of the body.
Learn hence, That although sin remaineth in him that is born of God, yet he that is born of God doth not remain in sin, either as to a sinful state, or a sinful conversation; God's word and Spirit, by which he was regenerate, still remain in him; and so far as he is under the ruling power and governing influences of them, he cannot sin, much less live in willful sin.
Burkitt, William. "Commentary on 1 John 3:9". Expository Notes with Practical Observations on the New Testament. https://www.studylight.org/commentaries/wbc/1-john-3.html. 1700-1703.