It'll happen when it's time. The only thing that "Theology" can produce is confusion, so neither MAN, or the Visible Churches will ever produce unity.
Back in 1973, I watched God bring all sorts of Churches together in a remarkable (but rather shallow) unity during the Charismatic outpouring which rolled on from about 1964 to 1979. I was in it from '72 through 78 or so. MANY PEOPLE were effected, and changed by it. ANd many denominations were changed. when the Charismatics started flowing back into the Assemblies of God denomination, it changed from a strongly legalistic, "Clothes-line holy", narrow minded, judgmental, bunch into a much more loving system, still maintaining it's strong missionary programs.
So when God decides it's TIME for the Churches to all see that same thing in Him - they will.