[the ancients] must have regarded the whole of Scripture as one book ... I will add to demonstrate this to you an apostolic utterance (ῥητὸν ἀποστολικὸν) not understood by the followers of Marcion, who therefore reject the Gospels (because of it). For whereas the Apostle says, "According to my gospel in Christ Jesus," and does not speak of gospels, they (the Marcionites) oppose us, and maintain that if there were several gospels the Apostle would not have used the word in the singular. They do not understand that as He is one, so the Gospel written by its many authors is one in effect, and that the true Diatessaron is one Gospel (καὶ τὸ ἀληθῶς διὰ τεσσάρων ἕν ἐστιν εὐαγγέλιον). John Chrysostom Homilies on Galatians:
the Marcionites are misled by this phrase (Gal 1.7 'another gospel') 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, [Romans 16:25] 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. And Paul is not now speaking of the number but of the discrepancy of the things spoken. With justice might they lay hold of this expression, if the Gospels of Matthew and Luke differed in the signification of their contents, and in their doctrinal accuracy; but as they are one and the same, let them cease being senseless and pretending to be ignorant of these things which are plain to the very children.Notice that in the standard translation "καὶ τὸ ἀληθῶς διὰ τεσσάρων" is translated "that truly delivered by four." But I wonder whether it is better translated "that the true Diatessaron" compare Plato Phaedo 109 E ὁ ἀληθῶς οὐρανὸς καὶ τὸ ἀληθῶς φῶς καὶ ἡ ὡς ἀληθῶς γῆ = "the real heaven and the real light and the real earth." So καὶ τὸ ἀληθῶς διὰ τεσσάρων ἕν ἐστιν εὐαγγέλιον would = that the real Diatessaron is one gospel. By the fifth century this was further corrupted into καὶ τὰ τεσσάρων Εὐαγγέλια ἕν ἐστιν Εὐαγγέλιον "that the four gospels is one gospel." But that wasn't there originally.