The Gregorian calendar is essentially non-Christian in origin. Yes, it is named for Pope Gregory, but he just tinkered with the leap year rule, other than that it is essentially the same as the Julian calendar, named for Julius Caesar

Now, the common AD/CE year numbering is obviously Christian in motivation, but that’s technically distinct from the Gregorian calendar-the Julian calendar historically used other year numbering systems too (ab urbe condita, anno Diocletiani aka anno martyrum, anno mundi), and the Gregorian calendar itself doesn’t care what numbers you assign to years beyond modulo 400 - you could replace 2025 with 2425 or 12025 and it would still work fine. But that’s the thing, Unix time doesn’t care whether 1970 is called 1970 or 1570 or 2370 or 11970, only how distant you are from it - so it isn’t really tied to our Christianity-inspired year numbering, only to 1970-01-01T00:00:00Z as an instant

I think CE + 10,000 - called “Holocene Era” (HE), also sometimes expanded Human Era or Historical Era - is a good proposal - high compatibility with CE (you can just pretend 2025 is missing the leading digit), makes all dates from recorded human history positive. Okay, is ambiguous in that 2025 CE might get confused with 2025 HE (= -7975 CE = 7976 BCE), but in practice that’s unlikely since that’s prehistorical and essentially nothing (outside astronomy) from that long ago can be dated to a year’s accuracy anyway