Notes from Mike Ferrando, with translation assistance from the helpful scholar in Thessalonika, Greece, Dr. Pavlos D Vasileiadis, Dec. 2018

A very good allusion to I John 5 7.
Forster's comments nail it.
Any bible reader and biblical Christian can see that this is an allusion to I John 5 7.
Caesarius (470-542 AD), Bishop of Arles : Forster Citation : GOD IS LIGHT : THREE IN ONE

===SOURCE:

Migne Graeca PG 38, 859-860 [860D]
https://books.google.com/books?id=1b...AJ&pg=RA1-PA78

S. Caesarius, B. Gregorii Frater (470-542 AD), Bishop of Arles
Dialogi quatuor, in quibus contineutur Explicationes Quaestionum quarumdam de rebus gravibus, Caesario propositarum, quo tempore a secretis erat, et Constantinopli docens.
Dialogus I : Interrogatio III

οὔτε αἱ τρεῖς ὑποστάσεις εἰς τοσαύτας φύσεις
τέμνουσι τὴν μίαν τὴς θεότητος οὐσίαν, οὔτε ἡ μία
οὐσία εἰς ἓν πρόσωπον καὶ μίαν ὑπόστασιν
συνελείφθη, καὶ συναιπεῖται τὴν τρίστομον καὶ
τρισαένναον κρήνην τῆς θεότητος·
φῶς τοίνυν ὁ Î*ατὴρ,
φῶς ὁ Υἱὸς,
φῶς τὸ θεῖον Î*νεῦμα·
ἀλλ’ οἱ τρεῖς ἓν ὑπάρχουσιν φῶς.

==

neither the three substances/hypostases in so many of natures
divide the one essence/ousia of the godhead, neither the one
essence/ousia in one person and one substance/hypostasis
is comprehended, and is inferred the three-edged and
the thrice ever-flowing fountain of the godhead;
therefore the Father is light,
the Son is light,
the divine Spirit is light;
but these three exist as one light.
===SOURCE:

A New Plea for the Authenticity of the Text of the Three Heavenly Witness; Or, Porson's Letters to Travis Eclectically Examined and the External and Internal Evidences for 1 John V, 7 Eclectically Re-surveyed
Author Charles Forster
Publisher Deighton, Bell, 1867
https://books.google.com/books?id=yXIsAAAAYAAJ&pg=PA92


[PAGE 92]
Let us descend from the fourth to the sixth century, and we find only increasing marks of designed reference to the disputed verse: thus writes St. Caesarius (470-542 AD), Bishop of Arles, A.D. 502:

οὔτε αἱ τρεῖς ὑποστάσεις εἰς τοσαύτας φύσεις
τέμνουσι τὴν μίαν τὴς θεότητος οὐσίαν, οὔτε ἡ μία
οὐσία εἰς ἓν πρόσωπον καὶ μίαν ὑπόστασιν
συνελείφθη, καὶ συναιπεῖται τὴν τρίστομον καὶ
τρισαένναον κρήνην τῆς θεότητος·
φῶς τοίνυν ὁ Î*ατὴρ,
φῶς ὁ Υἱὸς,
φῶς τὸ θεῖον Î*νεῦμα·
ἀλλ’ οἱ τρεῖς ἓν ὑπάρχουσιν φῶς.18

The THREE-ONE doctrine of the Godhead, as

[PAGE 93]
delivered in this passage, is precisely that revealed, contra distinctively, in 1 John v. 7, only here in reiterated terms. but this broad argument, palpable to every eye, is brought home to 1 John v. 7 by a single word, common to the two passages, namely St. John's definition of God, the term φῶς.

Of all the sacred writers of the New Testament, St. John alone defines God as LIGHT. The definition recurs seven times in his first Epistle. When, therefore, we read in St. Caesarius, at the close of a passage containing the whole substance of the unique seventh verse, ἀλλ’ οἱ τρεῖς ἓν ὑπάρχουσιν φῶς, his tacit quotation, in this passage, of the text of the three Heavenly Witnesses can be questioned by those only who do not choose to believe.

I would close with the remark that, not only is the definition of God as light peculiar to St. John, but that this definition is announced by the Apostle himself to be the very subject and substance of his First Epistle. "This, then, is the message which we have heard of Him, and declare unto you, that God IS LIGHT, and in Him is no darkness at all." I John i. 5.

==

18. This is St. John v. 7, verbatim et literatim, οἱ τρεῖς ἕν εἰσι. The substitution for εἰσι of ὑπάρχουσιν strengthens the note of quotation. It is Caesarius' reading of a text.