If, however, any one thinks to refute our argument on this ground, that even after the application of the remedial process the life of man is still in discord through its errors, let us lead him to the truth by an example taken from familiar things. Take, for instance, the case of a serpent; if it receives a deadly blow on the head, the hinder part of the coil is not at once deadened along with it; but, while the head is dead, the tail part is still animated with its own particular spirit, and is not deprived of its vital motion: in like manner we may see Sin struck its deadly blow and yet in its remainders still vexing the life of man.
But then they give up finding fault with the account of Revelation on these points, and make another charge against it; viz. that the Faith does not reach all mankind. “But why is it,” they ask, “that all men do not obtain the grace, but that, while some adhere to the Word, the portion who remain unbelieving is no small one; either because God was unwilling to bestow his benefit ungrudgingly upon all, or because He was altogether unable to do so?” Now neither of these alternatives can defy criticism.
For it is unworthy of God, either that He should not will what is good, or that He should be unable to do it. “If, therefore, the Faith is a good thing, why,” they ask, “does not its grace come upon all men?” Now, if in our representation of the Gospel mystery we had so stated the matter as that it was the Divine will that the Faith should be so granted away among mankind that some men should be called, while the rest had no share in the calling, occasion would be given for bringing such a charge against this Revelation.
But if the call came with equal meaning to all and makes no distinction as to worth, age, or different national characteristics (for it was for this reason that at the very first beginning of the proclamation of the Gospel they who ministered the Word were, by Divine inspiration, all at once enabled to speak in the language of any nation, viz. in order that no one might be destitute of a share in the blessings of evangelical instruction), with what reasonableness can they still charge it upon God that the Word has not influenced all mankind?
For He Who holds the sovereignty of the universe, out of the excess of this regard for man, permitted something to be under our own control, of which each of us alone is master. Now this is the will, a thing that cannot be enslaved, and of self-determining power, since it is seated in the liberty of thought and mind. Therefore such a charge might more justly be transferred to those who have not attached themselves to the Faith, instead of resting on Him Who has called them to believe.
For even when Peter at the beginning preached the Gospel in a crowded assembly of the Jews, and three thousand at once received the Faith, though those who disbelieved were more in number than the believers, they did not attach blame to the Apostle on the ground of their disbelief. It was, indeed, not in reason, when the grace of the Gospel had been publicly set forth, for one who had absented himself from it of his own accord to lay the blame of his exclusion on another rather than himself.
Source: The Great Catechism (New Advent)