It is quite in harmony with our conception of the original situation in Christian Egypt that the Gospel of the Hebrews clearly displays the heretical trademark. In the fragment preserved by Origen {c.240 AD}, Jesus declares (on an occasion that we can no longer recover with certainty): "Just now my mother, the Holy Spirit, siezed me by one of my hairs and carried me away to the high mountain Tabor." According to Cyril of Jerusalem {c.375 AD}, the following also stood in the Gospel of the Hebrews: "When Christ desired to come to earth to men, the good Father chose a mighty Power in heaven named Michael, and entrusted Christ to its care. And the Power entered the world and was called Mary, and Christ was in her womb seven months." The great importance which Michael has in the Egyptian magical texts -- Greek as as well as Coptic -- and in the Pistis Sophia is well known.
I'm considering the (Jewish) Melchizedekian turning, here.
Clement of Alexandria (c.200 AD) also draws on this source, 'Gospel to the Hebrews', which most probably was composed for the earliest Jewish-Christian community, before or at the time of Basilades (c.120 AD). The "mighty Power" chosen strongly evokes (is) Melchizedek, here as the Father/Son of God (analgous to the Teacher/Pupil) - an exquisitely metaphorical shift recorded in 'the Christ entrusted to Melchizedek' (Heb 1:5). Yet the Gospel of the Hebrews preserves the origins of this switcheroo: Melchizedek has been entirely erased, replaced by Michael, by that time; Sophia has been replaced by Mary, also. And this Power is bi-sexual - uh-oh! Jewish & Christian gnosticism, twined.
I'm focused on the date. That looks like an absolute terminus for the Melchizedek 'cult' in Alexandria probably two generations before c.120 AD: or c.60 AD.
In Epistle to the Hebrews, Apollos (presumably the Alexandrian) was preaching to a synagogue backsliding Melchizedekians c.55 AD, somewhere (perhaps Corinth) in Asia Minor; there's no Archangel Michael there, yet, at the time of Epistle (c.55 AD). But the Christ is now entrusted to Melchizedek, who should cede his place to the new Son. (This dogma would have been preached first in Alexandria before c.50 AD, if we follow Acts 18:24 correctly) Melchizedek is now supplanted, but only recently, hence the Melchizedek cult was on the wane. Yet the synthesis was incomplete in the minds of some Melchizedekian back-sliders in Asia Minor. We may conclude the Melchizedekian network (such as it had been: synagogues connected to an older, quasi-military Judeo-Egyptian fraternity) had collapsed. But the argument in Epistles to the Hebrews was too valuable to be lost, for the Eastern Mediterrean.
The promise of Rest (Heb 4:1-11) topically suggests the 'Gospel to Hebrews', also. I suspect the Gospel to the Hebrews went through varied iterations, composed after 80 AD, c.85-125 AD. It may have been too eclectic and inconsistent (multiple versions circulating), so heretical it was systematically destroyed by Church Fathers in the 3rd-5th C. and ever after.
These are just ideas I'm considering... In these documents, there were heretical/adaptive Jewish synagogues (Melchizedekians, suddenly open to radical cosmopolitan ideas) susceptible to a Christological belief, which became 'Jesus Christian' c.50-100 AD.