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. - Gleanings from the Writings of Baha'u'llah, p. 255. (80:2)

Progressive Revelation
A great stumbling block to many, in the way of religious unity, is the difference between the Revelations given by the different Prophets. What is commanded by one is forbidden by another; how then can both be right, how can both be proclaiming the Will of God? Surely the truth is One, and cannot change. Yes, the Absolute Truth is One and cannot change, but the Absolute Truth is infinitely beyond the present range of human understanding, and our conceptions of it must constantly change. Our earlier, imperfect ideas will be by the Grace of God replaced, as time goes on, by more and more adequate conceptions. Baha'u'llah says, in a Tablet to some Baha'is of Persia: (80:3)

O people! Words are revealed according to capacity so that the beginners may make progress. The milk must be given according to measure so that the babe of the world may enter into the Realm of Grandeur and be established in the Court of Unity. (80:4)

It is milk that strengthens the babe so that it can digest more solid food later on. To say that because one Prophet is right in giving a certain teaching at a certain time, therefore another Prophet must be wrong Who gives a different teaching at a different time, is like saying that because milk is the best food for the newborn babe, therefore, milk and nothing but milk should be the food of the grown man also, and to give any other diet would be wrong! 'Abdu'l-Baha says: (80:5)

Get Next Page