You missed the point of my post. CCC 1735 does not have anything to do with expressing the impossibility of a person in the state of grace obeying God’s Commandments. I am presuming for the moment, for the sake of argument, that such a person is indeed justified and in the state of grace, and therefore, can receive holy communion. The question is, if they are in the state of grace, how is it impossible for them to obey the moral commands of God?
I understood that point. The dogma excludes venial sins and venial sin may involve grave matter.
The dogma from Trent is this:
828 Can. 18. If anyone shall say that the commandments of God are even for a man who is justified and confirmed in grace impossible to observe: let him be anathema [cf. n. 804].
832 Can. 22. If anyone shall say that he who is justified can either persevere in the justice received without the special assistance of God, or that with that [assistance] he cannot: let him be anathema [cf. n. 804, 806].
804 But no one, however much justified, should consider himself exempt from the observance of the commandments [can. 20]; no one should make use of that rash statement forbidden under an anathema by the Fathers, that the commandments of God are impossible to observe for a man who is justified [can. 18 and 22: cf. n. 200]. “For God does not command impossibilities, but by commanding admonishes you both to do what you can do, and to pray for what you cannot do, and assists you that you may be able”; * “whose commandments are not heavy” [1 John 5:3], “whose yoke is sweet and whose burden is light” [Matt. 11:30]. For they who are the sons of God, love Christ: “but they who love him, (as He Himself testifies) keep his words” [John 14:23], which indeed with the divine help they can do. For although in this mortal life men however holy and just fall at times into at least light and daily sins, which are also called venial [can. 23], they do not for that reason cease to be just. For that word of the just, “Forgive us our trespasses” [Matt. 6:12; cf. n.107], is both humble and true. Thus it follows that the just ought to feel themselves more bound to walk in the way of justice, in that having been now “freed from sin and made servants of God” [Rom. 6:22], “living soberly and justly and piously” [Tit. 2:12], they can proceed onwards through Christ Jesus, through whom they “have access unto this grace” [Rom. 5:2]. For God “does not forsake those who have once been justified by His grace, unless He be first forsaken by them.” * And so no one should flatter himself because of faith alone [can. 9, 19, 20], thinking that by faith alone he is made an heir and will obtain the inheritance, even though he suffer not with Christ “that he may be also glorified” [Rom. 8:17]. For even Christ Himself (as the Apostle says), “whereas he was the Son of God, he learned obedience by the things which he suffered and being made perfect he was made to all who obey him the cause of eternal salvation” [Heb. 5:8 ff.] For this reason the Apostle himself admonishes those justified saying: “Know you not, that they who run in the race, all run indeed, but one receiveth the prize? So run, that you may obtain. I therefore so run, not as at an uncertainty, I so fight, not as one beating the air, but I chastise my body and bring it under subjection, lest perhaps when I have preached to others, I myself should become a castaway” [1 Cor. 9:24ff.]. So also the chief of the Apostles, Peter: “Labor the more, that by good works you may make sure your calling and election; for doing these things, you shall not sin at any time” [2 Pet. 1:10]. Thence it is clear that they are opposed to the teaching of orthodox religion who say that the just man sins at least venially in every good work [can. 25], or (what is more intolerable) that he merits eternal punishments; and that they also who declare that the just sin in all works, if in those works, in order to stimulate their own sloth and to encourage themselves to run in the race, with this (in view), that above all God may be glorified, they have in view also the eternal reward [can. 26, 31], since it is written: “I have inclined my heart to do thy justifications on account of the reward” [Ps. 118:112], and of Moses the Apostle says, that he “looked to the reward” [Heb. 11:26].
806 So also as regards the gift of perseverance [can. 16] of which it is written: He that “shall persevere to the end, he shall be saved” [Matt. 10:22; 24:13] (which gift cannot be obtained from anyone except from Him, “who is able to make him, who stands, stand” [Rom. 14:4], that he may stand perseveringly, and to raise him, who falls), let no one promise himself anything as certain with absolute certitude, although all ought to place and repose a very firm hope in God’s help. For God, unless men be wanting in His grace, as He has begun a good work, so will He perfect it, “working to will and to accomplish” [Phil. 2:13; can. 22]. * Nevertheless, let those “who think themselves to stand, take heed lest they fall” [1 Cor. 10:12], and “with fear and trembling work out their salvation” [Phil. 2:12] in labors, in watchings, in almsdeeds, in prayers and oblations, in fastings and chastity [cf. 2 Cor. 6:3 ff.]. For they ought to fear, knowing that they are born again “unto the hope of glory” [cf. 1 Rom. Pet. 1:3], and not as yet unto glory in the combat that yet remains with the flesh, with the world, with the devil, in which they cannot be victors, unless with God’s grace they obey the Apostle saying: “We are debtors, not to the flesh, to live according to the flesh. For if you live according to the flesh, you shall die. But if by the spirit you mortify the deeds of the flesh, you shall live” [Rom. 8:12 ff.].