Human nature lost, by original sin, the state of sanctity to which it had been elevated by God, and consequently was also deprived of the integrity and interior order which it possessed. Since then man lacks sufficient strength of will to fulfill all the moral precepts known to him. To do good became difficult after the appearance of sin on earth. And "this is what explains man's intimate division," the Second Vatican Council teaches. The whole of human life, individual and collective, is presented as a struggle, and indeed a dramatic one, between good and evil, between light and darkness.
God's help is absolutely necessary for us to carry out acts aimed at the supernatural life. It is not that we are capable of thinking something as our own, but that our capacity comes from God. Moreover, after the sin of origin, this help becomes even more necessary. "No one by himself and by his own strength frees himself from sin and rises above himself; no one is completely free from his weakness, or from his loneliness, or from his slavery"; we all have need of Christ, model, teacher, doctor, liberator, savior, life-giver. Without Him we can do nothing; with Him we can do everything.
Although human nature is not corrupted by sin of origin, we experience - even after Baptism - a tendency to evil and a difficulty in doing good: it is the so-called fomes peccati or concupiscence, which - without being in itself sin - proceeds from sin and is inclined to sin. Freedom itself, although not suppressed, is weakened.
We thus understand, in the light of this doctrine, that our good works, the fruits of holiness and apostolate, are in the first place God's; in the second place - very much in the second place - the result of having corresponded as instruments, always weak and disproportionate, of grace. The Lord asks us to always keep in mind the poverty of our condition, avoiding the danger of a fatuous vanity. For often," says St. Alphonsus Liguori, "the man dominated by pride is a worse thief than others, because he steals not earthly goods, but the glory of God (...). Indeed, according to the Apostle, by ourselves we cannot do a good work, nor even have a good thought (cf. 2 Cor. 3:5).... And since this is so, when we do any good, let us say to the Lord: We return to you, O Lord, what we have received at your hand (1 Chron 29:14)". This is what we should do with whatever fruit we find in our hands: offer it back to God, for we know that the evil, the deficiency, is ours; the beauty and the goodness are His.