That it is reasonably reckoned a Man's own Fault if he be not converted to God, although he cannot be converted without Grace

SINCE no one can be set on the way to his last end without the aid of divine grace, or without it have the necessary means of reaching that end, as are faith, hope, love and perseverance, some might think that man is not to blame for being destitute of these gifts, especially seeing that he cannot merit the assistance of divine grace, nor be converted to God unless God convert him: for none is responsible for that which depends on another. But allow this, and many absurdities follow. It follows that the man who has neither faith nor hope nor love of God, nor perseverance in good, still does not deserve punishment: whereas it is expressly said: He that believeth not the Son shall not see life, but the wrath of God abideth on him (John iii, 36). And since none reaches the end of happiness without the aforesaid endowments, it would follow further that there are some who neither attain to happiness nor yet suffer punishment of God: the contrary whereof is shown from what will be said to all present at the judgement of God: Come . . . . possess ye the kingdom prepared for you, or, Depart . . . . into everlasting fire (Matt. xxv, 34-41).

To solve this doubt, we must observe that though one can neither merit divine grace beforehand, nor acquire it by movement of his free will, still he can hinder himself from receiving it: for it is said of some: They have said unto God, 'Depart from us, we will not have the knowledge of thy ways' (Job xxi, 14). And since it is in the power of free will to hinder the reception of divine grace or not to hinder it, not undeservedly may it be reckoned a man's own fault, if he puts an obstacle in the way of the reception of grace. For God on His part is ready to give grace to all men: He wills all men to be saved and to come to the knowledge of the truth (1 Tim. ii, 4). But they alone are deprived of grace, who in themselves raise an obstacle to grace. So when the sun lights up the world, any evil that comes to a man who shuts his eyes is counted his own fault, although he could not see unless the sunlight first came in upon him.*

3.159 : How Man is delivered from Sin
3.161 : That a Man already in Mortal Sin cannot avoid more Mortal Sin without Grace