[There is] one body, and one Spirit, even as ye are called in one hope of your calling;
Note 7 at Eph 4:4: Disunity among believers was unacceptable to the Apostle Paul. He made this a major issue in his letter to the Corinthians. The reasons for his intolerance of disunity are listed in Eph 4:4-6.
We all are "one body" in Jesus Christ, we all have "one Spirit" giving us life, and we all have the same "hope" of seeing Christ and being transformed into His likeness (this verse). We all have "one Lord" who is our Master, and there is only "one faith" in Christ and His atonement, and "one baptism" into Jesus Christ (Eph 4:5). We are all one family because we all share "one God and Father" (Eph 4:6). One God is above all, through all (i.e., working through us), and in us all (indwelling every believer).