“One body and one Spirit, just as you were also called in one hope of your calling;”
Isn’t it interesting that immediately after the Apostle speaks of guarding the unity of the church, he starts inserting doctrine into the mix? As was alluded to already, there can be no unity unless there is maturity of faith and maturity of faith includes understanding the doctrines of Jesus Christ — so much so that the demonstration of one’s understanding is found in the way they live out our Lord’s commands.
The first part of this is a direct reference to the language Paul wrote to the Corinthian church in the twelfth chapter of his first letter. There is one body of Christ — his body is not divided between Jew and Gentile, slave and free. No, we are many individual persons saved by grace, united to local congregations, and bound together as one body if we are truly Christian. And, indeed, it is the Holy Spirit who does that binding (and gives the gifts to the church so that the church can function…noting that the gifts are given to the church — or, to understand the language from a different perspective, if you are outside of the church, you ought not expect such gifts to be present).
We have already discussed the nature of the hope of our calling as Paul has previously used this language (1:18). We are also reminded by Peter that we are to be able to defend this hope (1 Peter 3:15), which is the basis for apologetics. Here, again, Paul is setting forth some doctrinal principles that everyone in the church is called to hold onto and defend as part of what it means to be Christian. Unity of faith yields a unity in doctrine and vice versa. Such is how Paul begins laying forth this ancient confessional language.