Here is the notes from the original Douay Rheims Bible of 1609
6. Sins not. Jovinian and Pelagius falsely (as heretics used to do) argued upon these words and those that follow verse 9; the one, that the baptized could sin no more: the other, that no man being or remaining just, could sin. But among many good senses given of this place, this seems most agreeable, that the apostle should say, that mortal sin does not consist together with the grace of God, and therefore cannot be committed by a man continuing the son of God. And so is the like speech in the ninth verse following to be taken. See St. Jerome (li. 2. cont. Jovinia num, c. 1)
Cornelius a Lapide says this
**Ver. 9.—**And he cannot sin, because he is born of God. Hence Jovinian, Luther, and Calvin taught that a man could not fall away, but was sure of his salvation. But S. John says, “My little children, these things write I unto you, that ye sin not.” Consequently they could sin, faithful though they were. And it is contrary to daily experience, for we find daily the faithful becoming heretics and falling into sin. And the Council of Trent (vi. 23) rules otherwise. What then is S. John’s meaning that he who is born of God cannot sin, that is mortally and gravely? 1. We must take the word collectively—and then it will mean, So long as he preserves the seed of grace, he cannot sin. So Œcumenius, Thomas Anglicus, Cajetan, and S. Hierom, lib. 11 extra Jovin. And accordingly theologians say that he who has effectual grace cannot sin, because effectual grace in its very conception includes its result. For that grace is called ‘effectual’ which (as is foreseen) will produce its effect, which is to lead our free will to co-operate in a good work. But, speaking abstractedly, he who has effectual grace can resist it, and commit sin. (See Conc. Trid. sess. vi. can. 4.)
He who is born of God cannot (in a formal sense) commit sin, that is as far as relates to his heavenly new birth. For if this be allowed to act, and is not withstood by our free will, it is fully able to keep out all sin. (See S. Augustine, de grat. Christi, cap. xxi.) Thus Adam is said in his state of innocence to have been immortal, because he could not die, as long as he remained therein. But as he could fall, so also could he die. Thus we say that this medicine, e.g., is so powerful that any one who takes it could not die of the plague. But a man refuses to take the medicine and then dies; so can he who has the grace of God refuse to use it, and thus fall into sin. S. John here distinguishes between the supernatural action of Divine grace, and the exercise of moral virtues, the first of these preventing every sin, while the others do not. But the habit of temperance is not lost by one act of intemperance, even as temperance is not acquired by a single act of temperance. Again, the grace of Christ is distinguished from the grace given to Adam, which gave the power but not the will, whereas the grace of Christ gives both the will and the power. See S. Augustine (de corrupt. et gratia), “It is so provided (to meet the weakness of the human will), that Divine grace never fails, is never overpowered by any difficulty, so as ever to resolutely will that which is good, and obstinately refuse to abandon it.” And it is thus that he explains the words of S. John, “Every one that is born of God sinneth not.”
He cannot sin. He sins with difficulty. He has no wish to sin, says Œcumenius. Others explain the words, He has power not to sin, this power being given him by God.
Rightfully and properly he cannot sin, though he may in fact sin against all that is right and proper.
Gagneius says, “He cannot sin, i.e., by unbelief, which S. John calls a sin unto death.”
Some take these words as referring to those who are predestinated and absolutely elected to eternal life. But this must be understood, not of antecedent, but consequent impossibility, which consists with our liberty of will, as including and presupposing it.
The first and second of these explanations seem to be the best.