πρὸς ὃ δύνασθε ἀναγινώσκοντες νοῆσαι τὴν σύνεσίν μου ἐν τῷ μυστηρίῳ τοῦ Χριστοῦ.
When you read this, you can perceive my insight into the mystery of Christ.(ESV)
[With reference] to which, reading, you are able to know...(my overly literal rendition)
ὃ is the object of the preposition. That’s a relative pronoun, here declined as accusative*, neuter, singular.
- Accusative: This marks the use of πρὸς meaning “to” or “with regard to”. (See C.III.5 at the LSJ link or 3.e.δ. in BDAG.)
- Neuter, singular: Gender and number of the relative pronoun are normally governed by the antecedent. Here, the reference is to a previously written document alluded to in the verse 3. (“The mystery was made known to me by revelation, as I have written briefly.”) The neuter singular is frequently used for abstractions like this when there is no single word that can be identified as the antecedent.
Part of the trouble parsing this sentence may be with what follows — three verbal forms back-to-back. Keep δύνασθε + νοῆσαι together. The first is the finite verb (you are able), and νοῆσαι is an infinitive acting as its object/complement (to apprehend). The participle ἀναγινώσκοντες (reading) that is interjected between them is acting (to confuse English speakers and) as an adverb modifying that idea.**
*In your question you indicated that you thought it may be nominative. It is, in fact, morphologically identical to the nominative. Keep in mind that the nominative and accusative share a common form in the neuter gender. (I think that is always the case, but hopefully somebody will prove me wrong.)
On second thought, I wonder if you were thinking of it as the article
ὁ which does sometimes function as a pronoun and would be necessarily (masculine) nominative. Note the lack of an acute accent (vs.
ὃ in Eph 3:4, from the lexical form
ὅς). Very confusing!
**Possibly temporal (“as you read” - as indicated in the ESV quoted) although a “means” sort of interpretation also seems possible (“by reading”). Or some conglomeration of those two ideas.