Faustus argues that if the apostles born under the old covenant could lawfully depart from it, much more can he having been born a Gentile. Augustin explains the relation of Jews and Gentiles alike to the Gospel.
1. Faustus said: Another reason for not receiving the Old Testament is, that if it was allowable for the apostles, who were born under it, to abandon it, much more may I, who was not born under it, be excused for not thrusting myself into it. We Gentiles are not born Jews, nor Christians either. Out of the same Gentile world some are induced by the Old Testament to become Jews, and some by the New Testament to become Christians. It is as if two trees, a sweet and a bitter, drew from one soil the sap which each assimilates to its own nature. The apostle passed from the bitter to the sweet; it would be madness in me to change from the sweet to the bitter.
2. Augustin replied: You say that the apostle, in leaving Judaism, passed from the bitter to the sweet. But the apostle himself says that the Jews, who would not believe in Christ, were branches broken off, and that the Gentiles, a wild olive tree, were grafted into the good olive, that is, the holy stock of the Hebrews, that they might partake of the fatness of the olive. For, in warning the Gentiles not to be proud on account of the fall of the Jews, he says: "For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles. I magnify my office; if by any means I may provoke to emulation them which are my flesh, and might save some of them. For if the casting away of them be the reconciling of the world, what shall the receiving of them be, but life from the dead? For if the first fruit be holy, the lump is also holy; and if the root be holy, so are the branches. And if some of the branches are broken off, and thou, being a wild olive tree, were grafted in among them, and with them partakest of the root and fatness of the olive tree; boast not against the branches: but if thou boast, thou bearest not the root, but the root thee. Thou wilt say then, The branches were broken off, that I might be grafted in. Well; because of unbelief they were broken off, and thou standest by faith. Be not high-minded, but fear; for if God spared not the natural branches, take heed lest He also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in His goodness; otherwise thou also shalt be cut off. And they also, if they abide not still in unbelief, shall be grafted in; for God is able to graft them in again. For if thou wert cut out of the olive tree, which is wild by nature, and weft grafted contrary to nature into a good olive tree; how much more shall these, which be the natural branches, be grafted into their own olive tree? For I would not, brethren, that ye should be ignorant of this mystery (lest ye should be wise in your own conceits), that blindness in part is happened to Israel, until the fullness of the Gentiles be come in; and so all Israel shall be saved."1 It appears from this, that you, who do not wish to be graffed into this root, though you are not broken off, like the carnal unbelieving Jews, remain still in the bitterness of the wild olive. Your worship of the sun and moon has the true Gentile flavor. You are none the less in the wild olive of the Gentiles, because you have added thorns of a new kind, and worship along with the sun and moon a false Christ, the fabrication not of your hands, but of your perverse heart. Come, then, and be grafted into the root of the olive tree, in his return to which the apostle rejoices, after by unbelief he had been among the broken branches. He speaks of himself as set free, when he made the happy transition from Judaism to Christianity. For Christ was always preached in the olive tree, and those who did not believe on Him when He came were broken off, while those who believed were grafted in. These are thus warned against pride: "Be not high-minded, but fear; for if God spared not the natural branches, neither will He spare thee." And to prevent despair of those broken off, he adds: "And they also, if they abide not still in unbelief, shall be grafted in; for God is able to graft them in again. For if thou weft cut out of the olive tree, which is wild by nature, and wert grafted contrary to nature into a good olive tree, how much more shall these, which be the natural branches, be grafted into their own olive tree." The apostle rejoices in being delivered from the condition of a broken branch, and in being restored to the fatness of the olive tree. So you who have been broken off by error should return and be grafted in again. Those who are still in the wild olive should separate themselves from its barrenness, and become partakers of fertility.