There's a christianity.stackexchange.com, which specializes in these questions.

@1973 Thank you for the link, however the question involves both Christianity and other faiths, and asks for a historical event. I often see questions on this SE which are an overlap of different subjects, such as politics, so I assumed here was as good a fit as any.

A small quibble: the 1924 writing on the gnostic movement you have quoted predates the nag hammadi discoveries. I don’t believe the chronology supposed there stands up in the light of this later discovery.

The church is for syncretism when it is useful and against it when it is not.

@AllInOne Thanks, I neglected to check the publication date. It seems the Gnostic texts are from an earlier date. 100-300CE?

Exact parameters of the question aren''t clear to me at all. Pope Damasus I denounced various heresies, does that count?

@Brian Z It seems the heresies that Pope condemned were schisms, like two branches growing from the same stem. My question is only about syncretism, which is more like branches of separate trees crossing paths.

Title and body do not match & terminology needs focus: earliest action texts against syncretism is prior (50s & 100s) to 'holy see' 'Catholic church' & 'pope'; syncretism ≠ gnosticism (is that just an example or your prime interest?). Please clarify: earliest denouncing of overlap is in scripture, before 'the (universal) church' but from ''recognised' authority' in 'early church'; would that answer your Q?

@LangLangC Early Christianity is way more scrambled than I realized. A more specific date has been added, and the problematic Gnostic example removed.

It is possible that denunciation of Jesuits in China? There were some attempts at syncretism with Chinese religious practice. (from memory)

@Mark C. Wallace Since Jesuits are Christian then I don't know if that comparison would work. sorry for a poorly worded question, I'm unsure how to explain specifically the kind of syncretism I mean, beyond saying "non-Christian." Hence the examples (Mexican folk deity and Voodoo)

I'm very well aware that Jesuits are Christian; there were efforts to syncretize (?) with Tao ism/Buddhism. If I recall correctly, the early efforts were OK, but later the Pope came down against the efforts. ? Father Ricci???

@Mark C. Wallace my bad, I thought you just meant Jesuits. If the Pope condemned them for syncretization (whether attempted or successful) then that's exactly what I'm looking for.

May I suggest either the asker for further refinement or an answer writer to the first few pages of brill.com/view/title/34171?

In his biography of St. Augustine, Peter Brown recounts when that bishop denounced the North African custom of holding feasts at the gravesite of saints for being "pagan". But I'm sure there are earlier examples: one of the most vicious insults one Early Christian could hurl against another in the first centuries of this era was being called a heretic, a Jew... or a pagan.

The ones in favor of the Jesuit's Chinese rites argued that the rites were not syncretism with Budism, but were mostly civic in nature, or natural reverence to the ancestrals which would be compatible with Christian reverence and prayers for the dead. The contraries would disagree, or at least argue that the Chinese would not know the difference, and it would cause scandal anyway.

A missionary told me that in Cameroon, pagans could check a box on the civil marriage certificate to be allowed to wed a 2nd wife afterwards. And the church checks the certificates (usually the couple must marry civilly just before the church wedding) and does not wed them if the groom picked polygamy.

In religion schisms always develop, so this sort of thing undoubtedly predates Christianity.

Alexander Vi. Sometime in January 1500. Unless you require a documented instance. This is a bit like asking who was the first Ameeican president in the 20th century to make a reference to baseball.

The question is not about schisms. It’s about overlaps with non-Christian faiths.

Whoever is condemning someone else's belief will call the divergence heretical, not "they are Christians too, so it's all right".