Harmony of religions is the logical sequel to the first three cardinal principles of non-duality of the ultimate Reality, divinity of the soul, and oneness of existence.
Philosophy, mythology, and rituals are three aspects of every religion. In the words of Swami Vivekananda, “Philosophy...presents the whole scope of that religion, setting forth its basic principles, the goal and the means of reaching it. The second part is mythology, which is philosophy made concrete. It consists of legends relating to the lives of men, or of supernatural beings, and so forth....The third part is the ritual. This is still more concrete and is made up of forms and ceremonies, various physical attitudes, flowers and incense, and many other things that appeal to the senses."
Differences between religions are inevitable regarding these three aspects. Harmony or universality is possible only when we seek to realize the spiritual Truth behind all religions. We are no longer satisfied with the cursory aspects of religion and yearn to approach God, who dwells in our heart. We then appreciate the language of harmony spoken by the mystics of all religions. They care more for the kernel (the Truth) than the husk (the three aspects) of religion. We then understand that all religions lead us to the same Truth. Swami Vivekananda teaches that realization of the divinity in us and becoming free is the whole of religion. “Doctrines, or dogmas, or rituals, or books, or temples, or forms, are but secondary details." The secondary details are useful only to the extent they help us manifest our divinity.
As long as the secondary details of religion remain our main concern, differences with others are inevitable. But when we accept the idea that religion means realization of Truth and are earnest about spiritual disciplines, differences diminish, and we begin to manifest more and more of our hidden divine nature: we move closer to God. Swami Vivekananda explains: “There may be millions of radii converging towards the same centre in the sun. The further they are from the centre, the greater is the distance between any two. But as they all meet at the centre, all difference vanishes. There is such a centre, which is the absolute goal of mankind. It is God. We are the radii. The distances between the radii are the constitutional limitations through which alone we can catch the vision of God. While standing on this plane, we are bound each one of us to have a different view of the Absolute Reality; and as such, all views are true, and no one of us need quarrel with another. The only solution lies in approaching the centre. If we try to settle our differences by argument or quarrelling, we shall find that we can go on for hundreds of years without coming to a conclusion. History proves that. The only solution is to march ahead and go towards the centre; and the sooner we do that the sooner our differences will vanish."
According to Sri Ramakrishna, “All religions are true. The important thing is to reach the roof. You can reach it by stone stairs or by wooden stairs or by bamboo steps or by a rope. You can also climb up by a bamboo pole....It is enough to have yearning for God. It is enough to love Him and feel attracted to Him....All children are the same to the father. Likewise, the devotees call on God alone, though by different names. They call on one Person only. God is one, but His names are many."