There is a great difference between religion(s) and religion. One is the multiplicity of outward shapes, communities, and institutions. The other is the spirit that gives rise to them. From time to time the world must be shaken from the slumber of these established forms by the power that constituted them in the first place. Baha’is call this progressive revelation.
Religion, we might say, is the act of inquiring into the fundamental verities of the world and of our existence within it. Religion(s), on the other hand, function similarly to tribes or nations. A religion, we might say, is the population of its believers and the doctrines they espouse, the practices they observe, the culture they share. The first gives rise to a multiplicity of outward forms without sacrificing the singularity of its act. The latter is a great multiplicity whose diversified elements tend to be sacrificed if there is any attempt at unification. Each element is static and cannot pass over into something else without itself passing away.
Baha’u’llah teaches the unity of religion. All the world’s religions come from God. And they find their culmination in the Baha’i Revelation. This presents a difficult situation for Baha’is trying to explain this principle to others. For example, Christians eat pork. Muslims do not. But they do eat cows, which are revered by Hindus. Uniting these diversified elements into one religion would be absurd. One could say that it doesn’t matter what we eat, that such laws are bad for unity. But that would be to have the Christian view prevail to the detriment of all others. At the same time, people might suggest that Baha’i appreciation for other religions is just a ploy to get others to like the Baha’i Faith and eventually join it, to the detriment of their previous religion. In which case, Baha’is doesn’t seem to much appreciate other religions. After all, Baha’is would be trying to drain them of their adherents, which isn’t very nice. Unity of religion, then, would just be a bait and switch with little substantial meaning.
The reality is that Baha’u’llah did not accept the state of the world’s religions as it was, and does not want us to accept it as it is now. The outward forms enshrined by religions are inadequate and must be exposed to the power from which they are derived. He writes,
Verily, this is the Day in which mankind can behold the Face, and hear the Voice, of the Promised One. The Call of God hath been raised, and the light of His countenance hath been lifted upon men. It behoveth every man to blot out the trace of every idle word from the tablet of his heart, and to gaze, with an open and unbiased mind, on the signs of His Revelation, the proofs of His Mission, and the tokens of His glory.
The unity of the world’s religions is that they have been brought into the world by one spirit of divine revelation. It has from time to time been revealed to humanity giving laws and teachings for that day. And it has in this Day reappeared, first with the Bab, and then with Baha’u’llah. This is what unifies them. It challenges the adherents of all religions to turn to the animating impulse of their own faith and to examine whether or not, outside of their religion (one among others), Baha’u’llah is the return of the spirit that animates it in the first place.
 GWB VII pp. 10-11