Of the Catholic Church, the Remission of Sins, and the Resurrection of the Flesh
— Inasmuch, I repeat, as this is the case, we believe also in The Holy Church, [intending thereby] assuredly the Catholic. For both heretics and schismatics style their congregations churches. But heretics, in holding false opinions regarding God, do injury to the faith itself; while schismatics, on the other hand, in wicked separations break off from brotherly charity, although they may believe just what we believe. Wherefore neither do the heretics belong to the Church catholic, which loves God; nor do the schismatics form a part of the same, inasmuch as it loves the neighbor, and consequently readily forgives the neighbor's sins, because it prays that forgiveness may be extended to itself by Him who has reconciled us to Himself, doing away with all past things, and calling us to a new life. And until we reach the perfection of this new life, we cannot be without sins. Nevertheless it is a matter of consequence of what sort those sins may be.
22. Neither ought we only to treat of the difference between sins, but we ought most thoroughly to believe that those things in which we sin are in no way forgiven us, if we show ourselves severely unyielding in the matter of forgiving the sins of others. Thus, then, we believe also in The Remission of Sins.
23. And inasmuch as there are three things of which man consists—namely, spirit, soul, and body—which again are spoken of as two, because frequently the soul is named along with the spirit; for a certain rational portion of the same, of which beasts are devoid, is called spirit: the principal part in us is the spirit; next, the life whereby we are united with the body is called the soul; finally, the body itself, as it is visible, is the last part in us. This “whole creation” (creatura), however, “groans and travails until now.” Nevertheless, He has given it the first-fruits of the Spirit, in that it has believed God, and is now of a good will. This spirit is also called the mind, regarding which an apostle speaks thus: “With the mind I serve the law of God.” Which apostle likewise expresses himself thus in another passage: “For God is my witness, whom I serve in my spirit.” Moreover, the soul, when as yet it lusts after carnal good things, is called the flesh. For a certain part thereof resists the Spirit, not in virtue of nature, but in virtue of the custom of sins; whence it is said, “With the mind I serve the law of God, but with the flesh the law of sin.” And this custom has been turned into a nature, according to mortal generation, by the sin of the first man. Consequently it is also written in this wise, “And we were sometime by nature the children of wrath,” that is, of vengeance, through which it has come to pass that we serve the law of sin. The nature of the soul, however, is perfect when it is made subject to its own spirit, and when it follows that spirit as the same follows God. Therefore “the animal man receives not the things which are of the Spirit of God.” But the soul is not so speedily subdued to the spirit unto good action, as is the spirit to God unto true faith and goodwill; but sometimes its impetus, whereby it moves downwards into things carnal and temporal, is more tardily bridled. But inasmuch as this same soul is also made pure, and receives the stability of its own nature, under the dominance of the spirit, which is the head for it, which head of the said soul has again its own head in Christ, we ought not to despair of the restoration of the body also to its own proper nature. But this certainly will not be effected so speedily as is the case with the soul; just as the soul too, is not restored so speedily as the spirit. Yet it will take place in the appropriate season, at the last trump, when “the dead shall rise uncorrupted, and we shall be changed.” And accordingly we believe also in The Resurrection of the Flesh, to wit, not merely that that soul, which at present by reason of carnal affections is called the flesh, is restored; but that it shall be so likewise with this visible flesh, which is the flesh according to nature, the name of which has been received by the soul, not in virtue of nature, but in reference to carnal affections: this visible flesh, then, I say, which is the flesh properly so called, must without doubt be believed to be destined to rise again. For the Apostle Paul appears to point to this, as it were, with his finger, when he says, “This corruptible must put on incorruption.” For when he says this, he, as it were, directs his finger toward it. Now it is that which is visible that admits of being pointed out with the finger; since the soul might also have been called corruptible, for it is itself corrupted by vices of manners. And when it is read, “and this mortal [must] put on immortality,” the same visible flesh is signified, inasmuch as at it ever and anon the finger is thus as it were pointed. For the soul also may thus in like manner be called mortal, even as it is designated corruptible in reference to vices of manners. For assuredly it is “the death of the soul to apostatize from God;” which is its first sin in Paradise, as it is contained in the sacred writings.
Source: On Faith and the Creed (New Advent)