Eph 4:4-6, 4 There is one body and one Spirit, just as you were called in one hope of your calling; 5 one Lord, one faith, one baptism; 6 one God and Father of all, who is above all, and through all, and in you all.
Paul in these 3 verses makes a case for unity. In each verse he mentions one member of the Trinity. In verse 4 he begins with one body, in this one body there is one Spirit and one Lord (vs. 5), one God and Father of all (vs. 6), blessed Trinity. Believers who form the universal church (catholic), that is the one body.
In this one body, there are essential doctrines that must be the ground for our unity such as the Trinity (monotheism), the deity of Christ, Salvation by grace through faith in Jesus Christ alone, the resurrection of Christ, the gospel, creation, virgin birth, the return of Christ etc.
The absolute essentials are the Christian faith based on the doctrines of Christ. If the doctrines are removed, the faith is empty and void and there is no ground for unity. Paul in these verses again makes it clear that our unity is in one body, that of Trinity. He connects it with the following, his exhortation to strive for unity related to the Church, which is one body in Christ. The one Spirit forms it to the one hope which our calling rests on; to the one Lord, Christ, in whom you believe by faith, and above all, to the one God and Father.
In the essential doctrines we have a ground for unity, on the non-essentials we have in house debates while showing liberty and unity, but in all things we demonstrate charity (love).