Heidegger's Bible Handbook: John: The Occasion for Writing
4. A twofold occasion of writing is laid out, one out of Eusebius, the other out of Irenæus.
Ancient writers ascribe a twofold occasion for writing. The former: he, observing that by the three preceding Evangelists some things were written more obscurely, and those things that Christ preached and did before the delivering up of John, and were able to be explained and illustrated to advantage, were omitted, regarded it as necessary that the defects be supplied.That this was the occasion, Eusebius relates at length in Church History, book 3, section 24. Which, nevertheless, is tenuous enough, and has little weight, warns the Venerable Bullinger in his Præfatione in Johanne. The latter: the danger of the contagion of errors concerning the person and Divinity of Christ, introduced by Cerinthus in the time of John, and to be introduced thereafter by the foreseen cunning of Satan. That this was the occasion of the writing of the Gospel by Saint John, Irenæus alleges in contra hæreses, book III, section 12.Now, Cerinthus was teaching, as Irenæus testifies in book II, section 25, that Jesus was not born of a virgin, but that He was the son of Joseph and Mary, like all other men:that after His baptism the Christ descended upon Him, and then He proclaimed the Father, and performed miracles; that in the end the Christ departed from Jesus, and Jesus suffered and rose again. The same Irenæus thus writes in book III, section 12: John, desiring to exclude all heresies, and to establish a rule of truth in the Church, that there is one God omnipotent, who through His Word made all things, visible and invisible, etc., began thus:In the beginning, etc. Whatever the case may be, it is not able to be doubted that he, according to his most fervent faith in Christ, and his pious solicitude for the Church, desired especially to transcribe the clearly Divine and sublime discourses of Christ with the most exact care possible, so that from these the Church might learn unto the end of the world faith in Jesus Christ, true God and man, and love toward God and all the saints, and not be tossed by the uncertainty of opinion:and so a rule for most powerfully confuting all heresies, that were already existing at that time, and were going to exist thereafter, blasphemous teachings against Christ and His redemption, might be on record.