I came across these lines in a hymn:
Cherubim and seraphim falling down before Thee,
Which wert and art, and ever more shalt be.
I noticed that “wert”, “art”, and “shalt” were used with the subject “which” in the last line instead of which “thou.” At first I thought this was just a grammatical mistake on the side of the hymn writer, but then I kept seeing such things where verbs in second person singular form are used with indefinite pronouns such as “which” or “who”. Another example is give in this StackExchange question whose answer doesn’t really answer my question.
So now I’m wondering, is it correct to use second person singular verbs with indefinite pronouns if the indefinite pronoun refers to a second person singular pronoun (in the hymn, “which” reffers to “Thee” from the last line)?
Yes, “thou (…) who art” or “thee (…) who art” are correct.
I wasn’t sure from the title whether you were asking about relative pronouns or interrogative pronouns, so I will discuss both in my post.
In the hymn that you quote, the relative pronoun “which” takes second-person singular agreement because its antecedent is the second-person singular pronoun “thee”. This is a special thing that happened/happens in old-fashioned or formal English’ related questions about this topic are What rules make “Remember me, who am your friend” grammatical? and "Me who is" or "me who am"?
In terms of interrogatives, “Who art…” would be possible in a sentence with “thou”: “Who art thou?” In modern English, sentences of this type (e.g. “Who am I?”) are best analyzed as having “who” as the (fronted) predicate rather than as the subject: a piece of evidence that “I” and not “who” is the subject of “Who am I?” is that we can’t say *”Who am me”, even though in predicate position “me” is usually possible (we can say “It was me,” regardless of whether it’s considered “incorrect” from a prescriptive point of view). In older varieties of English, I’m not sure whether there is any clear way of establishing which word is the subject in questions like this.