The hypothesis was less violent in regard to the Synoptic Gospels, which clearly contain a large amount of common matter that might also have found its way into other hands.

We have evidence of the existence of other Gospels presenting a certain amount of affinity to the first Gospel, but the fourth is stamped with an idiosyncracy which makes it unique in its kind.

If there is to be this freedom in inventing unknown documents, reproducing almost verbatim the features of known ones, sober criticism is at an end.
That the Clementine Homilies imply the use of the fourth Gospel may be considered to be, not indeed certain in a strict sense of the word, but as probable as most human affairs can be.

The real element or doubt is in regard to their date, and their evidence must be taken subject to this uncertainty.
* * * * * It is perhaps hardly worth while to delay over the Epistle to Diognetus: not that I do not believe the instances alleged by Tischendorf and Dr.Westcott [Endnote 295:1] to be in themselves sound, but because there exists too little evidence to determine the date of the Epistle, and because it may be doubted whether the argument for the use of the fourth Gospel in the Epistle can be expressed strongly in an objective form.

The allusions in question are not direct quotations, but are rather reminiscences of language.