God can bring about spiritual maturity, and hence unity, through the spiritual leadership he gives to the church.
We’ve just been talking about unity in verses 1-6. Now Paul shows HOW Jesus has chosen to help us be unified: he gave certain people to serve the church by equipping EACH OF YOU to serve each other so that we all may grow to become mature, and therefore able to maintain unity in the Spirit due to our spiritual maturity.
The illustration here is that there are certain parts of the body that enable the other parts to grow. The mouth, for example, enables the rest of the body to consume food and receive nourishment. The hands accomplish all kinds of tasks that keep the rest of the body healthy and functioning.