The adjective Христов looks quite unique to a learner such as myself. I can't think of any others that look like it; meaning it lacks the characteristic -ый/ий/ой endings (and their inflected forms) I'm used to. It also seems to follow the declension pattern of a noun (or more specifically, male surnames, where the instrumental and all the plural forms are inflected like adjectives). I'd like to know why this is. Is it a grammatical holdover from Old Church Slavonic or Old East Slavic? If so, how did it come about, morphologically speaking? I'm only familiar with -ов in modern Russian, where it is an ending for plural nouns or singular male surnames. How was Христов formed from whatever the noun form of "Christ" was at the time?
I can't think of any others that look like it
Tons. Отцов, дедов, братов etc.etc.
I'm only familiar with -ов in modern Russian, where it is an ending for plural nouns or singular male surnames
Actually it's surnames which are produced from possessive forms, i.e. Петров means Peter's [son].
I was also surprised not to see "Христоса," the genitive of Христос, used instead
Genitive of Христос is Христа.
Is the -ов we see in possessive adjectives the same -ов we see in the inflectional ending for masculine nouns in the genitive plural today? Or is it etymologically unrelated?
These are different and do not always coincide. For example, братов (possessive) vs. братьев (gen. pl.).
I would have thought Петра would be "Peter's," whereas Петров would be "Peters'"
Петров = "Peter's" or "of Peters"; Петра = "of Peter".
Is it still a productive ending today
Yes, but it applies only to animate nouns, just like "'s" in English. Also Genitive is definitely more popular nowadays, which is especially true for "modern" words. So making possessive of such words is somewhere between "not recommended" and "wrong".
In Russian we have got possessive adjectives, which mean belonging to a person or an animal (singular ):
мамина сумка, отцовы кроссовки, кошачьи следы, нянькины сказки, кровь Христова братовы ботинки.
They are equal to a noun+noun structure
сумка мамы, кроссовки отца, следы кошки, etc.,
but this structure can show belonging to ownership in plural: ботинки братьев.That's why, they say,there's a tendency to use a noun+ noun structure or adjectives ending with -овский,ский more often.
The suffix -ов,(-ев) helps to form adjectives from nouns (masculine, ending with hard(soft) consonant
Отец – отцов, брат– братов, Христос – (we remove -ос) Христов, Христова, Христово.
We also remove -ос in declension :
Христос, Христа,Христу, Христа, Христом, о Христе. (храм Христа Спасителя).
Matt has already mentioned surnames, which were formed with the same suffixes.
To illustrate this short-form adjectival construction with last names in another setting, I have seen it used sometimes in math to name concepts after people: Артин --> артинов модуль (Artinian module), Риман --> риманова поверхность (Riemann surface), Банах --> банахово пространство (Banach space), and so on. It also occurs in physics: Ньютон --> ньютонова механика (Newtonian mechanics). If the name ends softly then -ов becomes -ев: Абель (Abel) --> абелев интеграл (abelian integral), абелева группа (abelian group), and абелевое многообразие (abelian variety). As other answers have indicated, this method of forming an adjective out of a name seems old-fashioned. Far more often I see the genitive following the noun, e.g., сфера Римана (Riemann sphere), not риманова сфера.
It is standard to use a lower-case first letter (артинов rather than Артинов, unless it is the first word of a sentence), and this short-form adjective precedes the noun. Neither of these is the case with your example of Христов; native speakers are welcome to comment on that.