Where would I ask a question about what significance certain words have in a old Spiritual song?
If the question you have in mind is an interpretation-like question, then Literature Stack Exchange would be a good fit. (This site did not yet exist when the previous answers were posted.)
If you know the language of the words and make it sufficiently narrow, such a question may be on-topic on the SE for the corresponding language, if one exists. If you do not know it, language-identification seems to be on-topic on Linguistics.
If your question is rather about the spiritual interpretation of the song, it may be on-topic on the SE for the respective religion, if one exists.
What of the above suggestions actually fits, if any, strongly depends on your actual question. So be careful to get an idea of the respective Stack Exchanges before asking, lest your question be closed.