My basic question [is] whether the "Jerusalem above" mentioned in Gal 4:26, which Thomas Aquinas identifiesBlosser:
with the Church militant (in the first place) and the Church triumpnant (in the second place), is a religious metaphor and, if so, how does Paul's speech function metaphorically. If Paul's words are not to be taken metaphorically, why not?
What difference would it make? Well, I am trying to see if viewing "Jerusalem above" as a metaphorical "mother" will shed any light on what we as Christians might mean when we address God as "Father." It appears to me that in both cases, "ontologization of gender" is or should be intended. Calling God "Father" does not address what He is immanently. It only has reference to what He is QUOAD NOS or PRO NOBIS.
I take it you intend "immanently" in the second-to-the-last sentence above as meaning "within God Him-self."
The key to this sort of business, where you're drawing a sort of boundary like Kant drew between that which is phenomenal (and apparant to us) and that which is noumenal (and lies beyond us), I should think, lies in whether or not we can sort out how metaphor can mean anything at all without meaning anything in God Himself. My hunch is that in many cases it may be mistaken to say that the metaphor means something only for us and not for God in Himself, because that may end up making the metaphor rather arbitrary.
My own approach to trying to answer this sort of problem would be to follow Wolterstorff's suggestions about objective qualities in things that make them
appropriate or inappropriate (fitting or unfitting) to serve as metaphors, via his notions of relative cross-modal similarity. Thus there are objective reasons why
"Behold the Lamb of God who takes away the sin of the world" is appropriately imputed to Jesus while "Behold the Piggy of God who takes away the sin of the world"