Objection 1. It would seem that man possessed of grace needs no help to persevere. For perseverance is something less than virtue, even as continence is, as is clear from the Philosopher (Ethic. vii, 7,9). Now since man is justified by grace, he needs no further help of grace in order to have the virtues. Much less, therefore, does he need the help of grace to have perseverance.
Objection 2. Further, all the virtues are infused at once. But perseverance is put down as a virtue. Hence it seems that, together with grace, perseverance is given to the other infused virtues.
Objection 3. Further, as the Apostle says (Romans 5:20) more was restored to man by Christ's gift, than he had lost by Adam's sin. But Adam received what enabled him to persevere; and thus man does not need grace in order to persevere.
On the contrary, Augustine says (De Persev. ii): "Why is perseverance besought of God, if it is not bestowed by God? For is it not a mocking request to seek what we know He does not give, and what is in our power without His giving it?" Now perseverance is besought by even those who are hallowed by grace; and this is seen, when we say "Hallowed be Thy name," which Augustine confirms by the words of Cyprian (De Correp. et Grat. xii). Hence man, even when possessed of grace, needs perseverance to be given to him by God.
I answer that, Perseverance is taken in three ways. First, to signify a habit of the mind whereby a man stands steadfastly, lest he be moved by the assault of sadness from what is virtuous. And thus perseverance is to sadness as continence is to concupiscence and pleasure, as the Philosopher says (Ethic. vii, 7). Secondly, perseverance may be called a habit, whereby a man has the purpose of persevering in good unto the end. And in both these ways perseverance is infused together with grace, even as continence and the other virtues are. Thirdly, perseverance is called the abiding in good to the end of life. And in order to have this perseverance man does not, indeed, need another habitual grace, but he needs the Divine assistance guiding and guarding him against the attacks of the passions, as appears from the preceding article. And hence after anyone has been justified by grace, he still needs to beseech God for the aforesaid gift of perseverance, that he may be kept from evil till the end of his life. For to many grace is given to whom perseverance in grace is not given.
Reply to Objection 1. This objection regards the first mode of perseverance, as the second objection regards the second.
Hence the solution of the second objection is clear.
Reply to Objection 3. As Augustine says (De Natura et Gratia xliii) [Cf. De Correp. et Grat. xii]: "in the original state man received a gift whereby he could persevere, but to persevere was not given him. But now, by the grace of Christ, many receive both the gift of grace whereby they may persevere, and the further gift of persevering," and thus Christ's gift is greater than Adam's fault. Nevertheless it was easier for man to persevere, with the gift of grace in the state of innocence in which the flesh was not rebellious against the spirit, than it is now. For the restoration by Christ's grace, although it is already begun in the mind, is not yet completed in the flesh, as it will be in heaven, where man will not merely be able to persevere but will be unable to sin.
Source: Summa Theologica (New Advent)