1 This truth, therefore, [he declares], in order that we may not reject the engrafting of the Spirit while pampering the flesh. “But you, being a wild olive-tree,” he says, “have been grafted into the good olive-tree, and been made a partaker of the fatness of the olive-tree.” As, therefore, when the wild olive has been engrafted, if it remain in its former condition, viz., a wild olive, it is “cut off, and cast into the fire;” but if it takes kindly to the graft, and is changed into the good olive-tree, it becomes a fruit-bearing olive, planted, as it were, in a king's park (paradiso): so likewise men, if they do truly progress by faith towards better things, and receive the Spirit of God, and bring forth the fruit thereof, shall be spiritual, as being planted in the paradise of God. But if they cast out the Spirit, and remain in their former condition, desirous of being of the flesh rather than of the Spirit, then it is very justly said with regard to men of this stamp, “That flesh and blood shall not inherit the kingdom of God;” just as if any one were to say that the wild olive is not received into the paradise of God. Admirably therefore does the apostle exhibit our nature, and God's universal appointment, in his discourse about flesh and blood and the wild olive. For as the good olive, if neglected for a certain time, if left to grow wild and to run to wood, does itself become a wild olive; or again, if the wild olive be carefully tended and grafted, it naturally reverts to its former fruit-bearing condition: so men also, when they become careless, and bring forth for fruit the lusts of the flesh like woody produce, are rendered, by their own fault, unfruitful in righteousness. For when men sleep, the enemy sows the material of tares; and for this cause did the Lord command His disciples to be on the watch. And again, those persons who are not bringing forth the fruits of righteousness, and are, as it were, covered over and lost among brambles, if they use diligence, and receive the word of God as a graft, arrive at the pristine nature of man— that which was created after the image and likeness of God.
2. But as the engrafted wild olive does not certainly lose the substance of its wood, but changes the quality of its fruit, and receives another name, being now not a wild olive, but a fruit-bearing olive, and is called so; so also, when man is grafted in by faith and receives the Spirit of God, he certainly does not lose the substance of flesh, but changes the quality of the fruit [brought forth, i.e.,] of his works, and receives another name, showing that he has become changed for the better, being now not [mere] flesh and blood, but a spiritual man, and is called such. Then, again, as the wild olive, if it be not grafted in, remains useless to its lord because of its woody quality, and is cut down as a tree bearing no fruit, and cast into the fire; so also man, if he does not receive through faith the engrafting of the Spirit, remains in his old condition, and being [mere] flesh and blood, he cannot inherit the kingdom of God. Rightly therefore does the apostle declare, “Flesh and blood cannot inherit the kingdom of God;” and, “Those who are in the flesh cannot please God:” not repudiating [by these words] the substance of flesh, but showing that into it the Spirit must be infused. And for this reason, he says, “This mortal must put on immortality, and this corruptible must put on incorruption.” And again he declares, “But you are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you.” He sets this forth still more plainly, where he says, “The body indeed is dead, because of sin; but the Spirit is life, because of righteousness. But if the Spirit of Him who raised up Jesus from the dead dwell in you, He that raised up Christ from the dead shall also quicken your mortal bodies, because of His Spirit dwelling in you.” And again he says, in the Epistle to the Romans, “For if you live after the flesh, you shall die.” [Now by these words] he does not prohibit them from living their lives in the flesh, for he was himself in the flesh when he wrote to them; but he cuts away the lusts of the flesh, those which bring death upon a man. And for this reason he says in continuation, “But if you through the Spirit do mortify the works of the flesh, you shall live. For whosoever are led by the Spirit of God, these are the sons of God.”
Source: Adversus haereses (New Advent)