Galatians 1:7

"Which is not another Gospel." And justly, for there is not another. Nevertheless the Marcionites are misled by this phrase, as diseased persons are injured even by healthy food, for they have seized upon it, and exclaim, "So Paul himself has declared there is no other Gospel." For they do not allow all the Evangelists, but one only, and him mutilated and confused according to their pleasure. Their explanation of the words, "according to my Gospel and the preaching of Jesus Christ," is sufficiently ridiculous; nevertheless, for the sake of those who are easily seduced, it is necessary to refute it. We assert, therefore, that, although a thousand Gospels were written, if the contents of all were the same, they would still be one, and their unity no wise infringed by the number of writers. So, on the other hand, if there were one writer only, but he were to contradict himself, the unity of the things written would be destroyed. For the oneness of a work depends not on the number of its authors, but on the agreement or contradictoriness of its contents. Whence it is clear that the four Gospels are one Gospel; for, as the four say the same thing, its oneness is preserved by the harmony of the contents, and not impaired by the difference of persons.

"Only there are some that trouble you, and would pervert the Gospel of Christ." That is to say, ye will not recognize another Gospel, so long as your mind is sane, so long as your vision remains healthy, and free from distorted and imaginary phantoms. For as the disordered eye mistakes the object presented to it, so does the mind when made turbid by the confusion of evil thoughts. Thus the madman confounds objects; but this insanity is more dangerous than a physical malady, for it works injury not in the regions of sense, but of the mind; it creates confusion not in the organ of bodily vision, but in the eye of the understanding. "And would pervert the Gospel of Christ." They had, in fact, only introduced one or two commandments, circumcision and the observance of days, but he says that the Gospel was subverted, in order to show that a slight adulteration vitiates the whole. For as he who but partially pares away the image on a royal coin renders the whole spurious, so he who swerves ever so little from the pure faith, soon proceeds from this to graver errors, and becomes entirely corrupted.

Where then are those who charge us with being contentious in separating from heretics, and say that there is no real difference between us except what arises from our ambition? Let them hear Paul's assertion, that those who had but slightly innovated, subverted the Gospel. Not to say that the Son of God is a created Being, is a small matter. Know you not that even under the elder covenant, a man who gathered sticks on the sabbath, and transgressed a single commandment, and that not a great one, was punished with death? and that Uzzah, who supported the Ark when on the point of being overturned, was struck suddenly dead, because he had intruded upon an office which did not pertain to him? Wherefore if to transgress the sabbath, and to touch the falling Ark, drew down the wrath of God so signally as to deprive the offender of even a momentary respite, shall he who corrupts unutterably awful doctrines find excuse and pardon? Assuredly not. A want of zeal in small matters is the cause of all our calamities; and because slight errors escape fitting correction, greater ones creep in.