By Sant Kirpal Singh, from the book "Spirituality – what it is", chapter 2

All apparent religious differences are man-made and are the result of narrowness and bigotry.

Saints and seers have one common message for the entire world. Their message is one of universal love. No one, indeed, can prove his love of God unless he knows how to love his fellow beings. Just as physical maladies wreck the human body, so do mental perversities. The latter so poison the circulatory system in the body, that one is badly affected by greed, selfishness, hatred, ill will and animosity, which in turn lead to perverted outlook on life. Thus man is dragged down to the level of beasts, nay, at times, man descends even lower than the beasts. Very often, the result is social and economic disintegration.

Whenever Master-souls come into the world, they tell us that all religious differences are the outcome of ignorance, individual whim, religious vanity or spiritual egotism. So-called leaders in every religion suffer from misguided fervour and narrow prejudices, so that they cannot possibly take a detached view of things around them. On the contrary, they see the world through the smoke-coloured glasses they have provided for themselves. They have no toleration for things and conditions that are not in accord with the rigid creeds of their organized sectarian or religious orders.

There is only one world-embracing universal religion, the religion of love, based on the great fundamental truth – the Fatherhood of God and the brotherhood of man, We have through self-interest, petty, prejudices and befogged understanding, carved out narrow sectarian principalities. We have raised around us hedges and walls of hatred and antagonism thus dividing man from man, class from class, nation from nation, and country from country. In this connection, Hafiz, a great Mohammedan saint, stated:

One reality shines both in Islam and Kufar (the man of faith, and a heretic)
and all the seeming differences in the various orders are, in fact,vapourous nothings.
It is through sheer prejudice that the Brahmans and the Sheikhs (religious heads
of Hinduism and Islam, respectively) have now different drinking bowls,
though in a wine-bar there is only one butler (the God-man), dispensing the
same wine (of divine love) from the same flagon to the various tipplers at the table.

Saints tell us that there is only one God of the entire universe. The Upanishads say the same thing, "Truth is one, though sages call it variously." He is the God of the whole creation and not of one religion or the other. There is, in fact, no difference between Karta (the one true creator) of the Hindus and Karim (merciful) of the Muslims, Ram (sustainer of the Hindus) and Rahim (compassionate of the Muslims). All these names are descriptive of the various attributes of God and were coined by sages, saints, rishis and munis of different denominations in their own respective languages. The nameless reality is one but responds to the calls of all by any name by which any individual may invoke that power.

The nameless one has many names. He doth attend, by whatsoever name
     He is addressed. – Maulvi Rumi

One must carefully avoid the dangers of doubt and skepticism. God alone need be worshipped and adored. He is the God of all, and each one is His manifestation. The same life-impulse works in all, and each is lighted by the same light. The entire humanity constitutes a single class by itself. Guru Gobind Singh says in this context:

"Some keep shaven crowns, while others robe themselves in flame-coloured apparel, and still others call themselves Jogis (a sect that wear wooden earrings in their ears and are ever on the march from stage to stage). Again, some are observing celibacy in quest of the Lord, while others perform penances and strict austerities. Some are Hindus and some are Turks, while others are Imams, Rafzi or followers of any other saint. With all these differences in nomenclature, they at the root are all one – mankind – to wit, men born of and embedded in God. Call Him the creator, the merciful, the donor or Rahim, for that makes no difference at all – take this as a settled truth and be not therefore bewildered by diversity in names. They all serve and worship the same God, the same Lord and Master of the universe. All of them manifest the image of the same God and exist simply by His love and light alone. Many a name has the nameless one; addressed in any, He doth attend."