In Ephesians 4:1-6 Paul writes, "I therefore, a prisoner for the Lord, urge you to walk in a manner worthy of the calling to which you have been called, with all humility and gentleness, with patience, bearing with one another in love, eager to maintain the unity of the Spirit in the bond of peace. There is one body and one Spirit—just as you were called to the one hope that belongs to your call — one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all" (emphasis mine).
In Paul's letter to the Ephesians, the apostle (to be simplistic about it) writes three chapters telling us what God has done for us, and then writes three chapters telling us how to live out our lives in Christ.
The above passage comes from the beginning of chapter four. Thus, this is the first thing Paul had to say to the Ephesians about how they were to live together after being saved. Interestingly, Paul chooses to emphasize the importance of unity. In the above passage he uses the word "one" eight different times. He also writes that they are to be eager to maintain the unity of the Spirit.
As there is one God, as we have one hope, as we have one faith, we also should be part of one body.
As the Trinity is united, we should be united (see John 17 for more on that).
Paul is clear: the church is to be united. There is no room for disharmony and division.