102 Even in the most difficult situations man must respect the norm of morality so that he can be obedient to God's holy commandment and consistent with his own dignity as a person. Certainly, maintaining a harmony between freedom and truth occasionally demands uncommon sacrifices, and must be won at a high price: it can even involve martyrdom. But, as universal and daily experience demonstrates, man is tempted to break that harmony: "I do not do what I want, but I do the very thing I hate... I do not do the good I want, but the evil I do not want" ( Rom 7:15, 19). What is the ultimate source of this inner division of man? His history of sin begins when he no longer acknowledges the Lord as his Creator and himself wishes to be the one who determines, with complete independence, what is good and what is evil. "You will be like God, knowing good and evil" ( Gen 3:5): this was the first temptation, and it is echoed in all the other temptations to which man is more easily inclined to yield as a result of the original Fall. But temptations can be overcome, sins can be avoided, because together with the commandments the Lord gives us the possibility of keeping them: "His eyes are on those who fear him, and he knows every deed of man. He has not commanded any one to be ungodly, and he has not given any one permission to sin" ( Sir 15:19-20). Keeping God's law in particular situations can be difficult, extremely difficult, but it is never impossible. This is the constant teaching of the Church's tradition, and was expressed by the Council of Trent: "But no one, however much justified, ought to consider himself exempt from the observance of the commandments, nor should he employ that rash statement, forbidden by the Fathers under anathema, that the commandments of God are impossible of observance by one who is justified. For God does not command the impossible, but in commanding he admonishes you to do what you can and to pray for what you cannot, and he gives his aid to enable you. His commandments are not burdensome (cf. 1 Jn 5:3); his yoke is easy and his burden light (cf. Mt 11:30)".
Source: Veritatis Splendor (Vatican.va)