Summa contra Gentiles3.155 Chapter CLV. That man needs the divine aid in order to persevere in good

Man also needs the aid of divine grace in order to persevere in good.

Because everything that is changeable of itself, needs the aid of an immovable mover, in order to stand fast to one thing. Now, man is changeable from evil to good, and from good to evil. Therefore, that he may persevere unchangeably in good, in a word, that he may persevere, he needs the divine assistance.

Again. Man needs the aid of divine grace for that which exceeds the strength of free-will. But the free-will is not sufficiently strong to persevere in good to the end. This is proved as follows. The power of the free-will extends to things which are a matter of choice: and that which is chosen is some particular thing to be done. A thing to be done in particular is something here and now. Therefore the power of the free-will is confined to something to be done now. But perseverance does not denote something to be done now, but a continuous operation that lasts all the time. Consequently this effect, namely to persevere in good, is above the power of the free-will. Therefore man needs the aid of divine grace, to persevere in good.

Besides. Although, by his will and faculty of self-determination, man is master of his actions, he is not master of his natural powers. And, consequently, though he is free to will or not to will a thing, in willing he is unable to make his will so will a thing that it stands fast to what it wills or chooses. Yet this is requisite for perseverance: namely, that the will stand fast in the good. Therefore perseverance is not in the power of the free-will: and consequently man needs the help of divine grace, in order to persevere.

Moreover. If there be several successive agents, one of which, namely, acts after the action of another: the continuation of their activity cannot result from any one of them, since none of them is always in action: nor can it result from them all, because they do not act together. Consequently it must result from some higher agent that is always in action. Thus the Philosopher (8 Phys. vi.) proves that the continuity of generation in animals is caused by some everlasting higher being. Now, let us suppose a man to be persevering in good. It follows that in him there are many movements of the free-will tending to the good, one after the other, until the end. Hence no one of these movements can be the cause of this continuity of good, namely perseverance; because none of them lasts continually. Nor can they all together be the cause of it: since, as they are not together, they cannot together be the cause of anything. Therefore this continuity is caused by some higher being: and consequently man needs the aid of grace from above in order to persevere in good.

Further. If many things be ordered to one end, their whole order, until they reach the end, is from the first agent directing them to the end. Now, in the man who perseveres in good there are many movements and actions tending to one end. Consequently the whole order of these movements and actions must needs be directed by the first director to that end. But it has been shown above that men are directed to the last end by the aid of divine grace. Therefore in him who perseveres in good the whole order and continuation of good works is through the assistance of divine grace.

Hence it is said (Philip, i. 6): He who hath begun a good work in you will perfect it unto the day of Christ Jesus: and (1 Pet. v. 10): The God of all grace, who hath called us unto His eternal glory... after you have suffered a little, will Himself perfect you, and confirm you, and establish you.

Moreover we find in Holy Writ many prayers by which perseverance is besought of God, for instance in Ps. xvi. 5, Perfect thou my goings in Thy paths, that my footsteps be not moved; and 2 Thess. ii. 15, 16, God our Father... exhort your hearts, and confirm you in every good work and word. The same petition is made in the Lord’s prayer, especially when we say, Thy kingdom come: for God’s kingdom will not come to us, except we persevere in good. Now, it would be absurd to ask of God that whereof He is not the giver. Therefore man’s perseverance is from God.

Hereby we refute the error of the Pelagians, who said that the free-will suffices for man to persevere in good, and that he needs not the assistance of grace in order to do so.

It must be observed, however, that since even one who has grace, asks of God that he may persevere in good; just as the free-will is not sufficient for this effect which is perseverance in good, without the external aid of God, so neither is an infused habit sufficient for the purpose. Because the habits that are infused into us by God, in the present state of life, do not wholly remove from the free-will its proneness to evil: albeit they do give the free-will a certain stability in good. Hence, when we say that man needs the help of grace for final perseverance, we do not mean that, in addition to habitual grace previously infused in order that he may do good works, he needs another grace besides in order to persevere: but we mean that even when he has all the infused gratuitous habits, man still needs the assistance of divine providence governing him externally.