What I meant by that is that Biblical references are well known. That is not to say that the author(s) of the Bible is(are) well known. That, to put it mildly, is far from a settled matter and will probably be debated to the end of time.
But to your "word of God" angle comment. Personally - I do come from a word of God angle. But my preference for a service such as MusicBrainz is to not put anyone in a position of supporting things they disagree with. These attributes are supposed to be statements of fact. Dispassionate and verifiable. Categorizing scripture as anonymous or unknown will put many into a position of supporting data they don't believe to be facts. I'm sure the number of people in that category is a very small minority.
But that is the fundamental reason why I would prefer a construct which avoids the issue entirely. I would prefer to not have to say [anonymous] or [unknown]. But something else. That something else should not be something that other people also find trouble with. But perhaps that cannot be.
From that perspective I prefer [unknown] of the existing choices because I find it the least objectionable.