I have not given up on the notion - or the apparent biblical imperative - for UNITY among the Body of true believers. But I am increasingly unconvinced that anything much more than occasional, superficial, temporary, localized unity can ever happen in greater "Christendom."
The more I meditate on it, and the more I observe the outward, visible manifestations of what the world knows to be The Church, the more I accept the probability that Ephesians 4:13 is not directed at any hope other than the inescapable spiritual UNITY of all those who have given themselves over completely to unabashed joy in celebrating the sovereignty and the mitzvot of Yhvh.
Portions of The Church have tried in vain to capture and sustain unity based on "central truths" while being tolerant of each others' divergent interpretations of Torah. Focusing on common denominators (what an appropriate term!) will never give rise to the spiritual convergence of Yhvh's people. Only when His people seek Him and yearn for an intimate personal relationship with Him that will motivate them to worship Him in spirit and in truth will we begin to approach UNITY; because when we are all focused on HIM ALONE, we will BE ONE.