In my personal opinion I think that the greatest barrier to unity is the existence of all the older religions who all cling to their own beliefs.
There can never be unity until the adherents to the older religions are willing to unite under one common banner.
I am not worried because I believe that will happen eventually, since it has been ordained by God.
“That which the Lord hath ordained as the sovereign remedy and mightiest instrument for the healing of all the world is the union of all its peoples in one universal Cause, one common Faith. This can in no wise be achieved except through the power of a skilled, an all-powerful and inspired Physician. This, verily, is the truth, and all else naught but error.” The Summons of the Lord of Hosts, p. 91