1 Among the other [truths] proclaimed by the apostle, there is also this one, “That flesh and blood cannot inherit the kingdom of God.” This is [the passage] which is adduced by all the heretics in support of their folly, with an attempt to annoy us, and to point out that the handiwork of God is not saved. They do not take this fact into consideration, that there are three things out of which, as I have shown, the complete man is composed — flesh, soul, and spirit. One of these does indeed preserve and fashion [the man]— this is the spirit; while as to another it is united and formed— that is the flesh; then [comes] that which is between these two— that is the soul, which sometimes indeed, when it follows the spirit, is raised up by it, but sometimes it sympathizes with the flesh, and falls into carnal lusts. Those then, as many as they be, who have not that which saves and forms [us] into life [eternal], shall be, and shall be called, [mere] flesh and blood; for these are they who have not the Spirit of God in themselves. Wherefore men of this stamp are spoken of by the Lord as “dead;” for, says He, “Let the dead bury their dead,” because they have not the Spirit which quickens man.
2. On the other hand, as many as fear God and trust in His Son's advent, and who through faith do establish the Spirit of God in their hearts—such men as these shall be properly called both “pure,” and “spiritual,” and “those living to God,” because they possess the Spirit of the Father, who purifies man, and raises him up to the life of God. For as the Lord has testified that “the flesh is weak,” so [does He also say] that “the spirit is willing.” For this latter is capable of working out its own suggestions. If, therefore, any one admix the ready inclination of the Spirit to be, as it were, a stimulus to the infirmity of the flesh, it inevitably follows that what is strong will prevail over the weak, so that the weakness of the flesh will be absorbed by the strength of the Spirit; and that the man in whom this takes place cannot in that case be carnal, but spiritual, because of the fellowship of the Spirit. Thus it is, therefore, that the martyrs bear their witness, and despise death, not after the infirmity of the flesh, but because of the readiness of the Spirit. For when the infirmity of the flesh is absorbed, it exhibits the Spirit as powerful; and again, when the Spirit absorbs the weakness [of the flesh], it possesses the flesh as an inheritance in itself, and from both of these is formed a living man—living, indeed, because he partakes of the Spirit, but man, because of the substance of flesh.
3. The flesh, therefore, when destitute of the Spirit of God, is dead, not having life, and cannot possess the kingdom of God: [it is as] irrational blood, like water poured out upon the ground. And therefore he says, “As is the earthy, such are they that are earthy.” But where the Spirit of the Father is, there is a living man; [there is] the rational blood preserved by God for the avenging [of those that shed it]; [there is] the flesh possessed by the Spirit, forgetful indeed of what belongs to it, and adopting the quality of the Spirit, being made conformable to the Word of God. And on this account he (the apostle) declares, “As we have borne the image of him who is of the earth, we shall also bear the image of Him who is from heaven.” What, therefore, is the earthly? That which was fashioned. And what is the heavenly? The Spirit. As therefore he says, when we were destitute of the celestial Spirit, we walked in former times in the oldness of the flesh, not obeying God; so now let us, receiving the Spirit, walk in newness of life, obeying God. Inasmuch, therefore, as without the Spirit of God we cannot be saved, the apostle exhorts us through faith and chaste conversation to preserve the Spirit of God, lest, having become non-participators of the Divine Spirit, we lose the kingdom of heaven; and he exclaims, that flesh in itself, and blood, cannot possess the kingdom of God.
4. If, however, we must speak strictly, [we would say that] the flesh does not inherit, but is inherited; as also the Lord declares, “Blessed are the meek, for they shall possess the earth by inheritance;” as if in the [future] kingdom, the earth, from whence exists the substance of our flesh, is to be possessed by inheritance. This is the reason for His wishing the temple (i.e., the flesh) to be clean, that the Spirit of God may take delight therein, as a bridegroom with a bride. As, therefore, the bride cannot [be said] to wed, but to be wedded, when the bridegroom comes and takes her, so also the flesh cannot by itself possess the kingdom of God by inheritance; but it can be taken for an inheritance into the kingdom of God. For a living person inherits the goods of the deceased; and it is one thing to inherit, another to be inherited. The former rules, and exercises power over, and orders the things inherited at his will; but the latter things are in a state of subjection, are under order, and are ruled over by him who has obtained the inheritance. What, therefore, is it that lives? The Spirit of God, doubtless. What, again, are the possessions of the deceased? The various parts of the man, surely, which rot in the earth. But these are inherited by the Spirit when they are translated into the kingdom of heaven. For this cause, too, did Christ die, that the Gospel covenant being manifested and known to the whole world, might in the first place set free His slaves; and then afterwards, as I have already shown, might constitute them heirs of His property, when the Spirit possesses them by inheritance. For he who lives inherits, but the flesh is inherited. In order that we may not lose life by losing that Spirit which possesses us, the apostle, exhorting us to the communion of the Spirit, has said, according to reason, in those words already quoted, “That flesh and blood cannot inherit the kingdom of God.” Just as if he were to say, “Do not err; for unless the Word of God dwell with, and the Spirit of the Father be in you, and if you shall live frivolously and carelessly as if you were this only, viz., mere flesh and blood, you cannot inherit the kingdom of God.”
Source: Adversus haereses (New Advent)