One Lord, one faith, one baptism

3 eager to maintain the unity of the Spirit in the bond of peace. 4 There is one body and one Spirit—just as you were called to the one hope that belongs to your call— 5 one Lord, one faith, one baptism (Eph 4:3-5).

This is a Catholic prooftext, so I'd like to discuss it:

i) The source of basis of unity isn't the church, the papacy, the magisterium, but the Holy Spirit. Of course, Catholics will say that's channeled through the magisterium, but they can't get that from this text (or any text!)

ii) Even in the mid-1C, when this letter was written, there were many local churches, so a plurality of churches in consistent with "one body".

In addition, the unity stands in contrast to pre-Christian divisions. Ancient tribal and ethnic rivalries and animosities. The 1C church was geographically and demographically diverse. Christians in Israel, Syria, Greece, Rome, &c. Jews, Gentiles, patricians, plebeians, slaves, men, women, &c. The Christian faith incorporated these disparate and competitive people-groups and social classes into the family of faith.

iii) "One Lord" refers to Jesus. All Christians have the same Lord. Incidentally, "One Lord" applies the Shema to Jesus. A prooftext for the deity of Christ.

iv) "One faith" could either denote objective faith or subjective faith. If the former, it refers to the apostolic kerygma. All Christians share that common frame of reference.

If the latter, it refers to the exercise of faith. But there's no much practical difference between the two inasmuch as the object of faith is the apostolic kerygma.

v) "Baptism" is ambiguous. It could denote water baptism. But unlike the Gospels and Acts, where the narrative setting clarifies the reference to water baptism, passages about "baptism" in the epistles usually lack that context.

It might denote Spirit-baptism (e.g. 1 Cor 12:13). That's something all Christians share in common.

Or it could be a metaphor (e.g. 1 Cor 10:2). At this early stage in Christian theology, we should guard against the anachronistic assumption that "baptism" was already a technical term for the rite of initiation. Usage may not have hardened yet.

So there's nothing in this passages that's at odds with Protestant theology or denominations. 

Catholic apologists might complain that this is just my private interpretation, and the ambiguities of the passage, which give rise to multiple interpretive options, demonstrate the need for a divine teaching office. To that I'd say two things:

i) Assuming the magisterium, you could, in theory, appeal to the magisterium to resolve these ambiguities–but it's premature at this stage of the argument to invoke the magisterium when this is supposed to be a prooftext for the magisterium. It would be viciously circular for a Catholic apologist to appeal to magisterial authority at this preliminary juncture when he's using this text to establish magisterial authority in the first place. 

ii) To my knowledge, Rome has never even purported to present an official interpretation of this passage.