Many religious theories are based on belief. One man says there is a great Being sitting above the clouds and governing the whole universe, and he asks me to believe that solely on the authority of his assertion. In the same way, I may have my own ideas, which I am asking others to believe, and if they ask a reason, I cannot give them any. This is why religion and metaphysical philosophy have a bad name these days. Every educated person seems to say, "Oh, these religions are only bundles of theories without any standard to judge them by, all preaching their own pet ideas."
Nevertheless, there is a basis of universal belief in religion, governing all the different theories and all the varying ideas of different sects in different countries. Going to their basis we find that they also are based upon universal experiences.
From Swamiji's book "Raja Yoga," Chapter One, "Introductory." Complete Works, 1.125-26.
Be not negative. You must be all-positive, affirmative. Say, "I am, God is, everything is in me. I will manifest health, purity, knowledge, whatever I want."
Letter to his brother-disciples. Written in Bengali from New York on September 25, 1894. Complete Works, 6.276.
When nature shines, upon what depends its shining? Upon God and not upon the sun, nor the moon, nor the stars. Wherever anything shines, whether it is the light in the sun or in our own consciousness, it is God. He shining, all shines after him.
From a lecture on "What Is Religion?". Complete Works, 1.337. Here Swamiji is paraphrasing from the Kaṭha Upaniṣad 2.2.15 and the Muṇḍaka Upaniṣad 2.2.10.
Why do you not preach Sri Ramakrishna as an Avatar?
Swamiji: Frankly, I have understood him very little. He appears to me to have been so great that, whenever I have to speak anything of him, I am afraid lest I ignore or explain away the truth, lest my little power does not suffice, lest in trying to extol him I present his picture by painting him according to my lights and belittle him thereby!
Conversations recorded in Bengali. From the diary of Sarat Chandra Chakravarty. Complete Works, 5.390.
If you analyze all the various religions of the world, you will find that ... the truths they teach are the results of the experiences of particular persons. The Christian asks you to believe in his religion, to believe in Christ and to believe in him as the incarnation of God, to believe in a God, in a soul, and in a better state of that soul. If I ask him for reason, he believes in them. But if you go to the fountainhead of Christianity, you will find that it is based on experience. Christ said he saw God; the disciples said they felt God; and so forth.
Similarly, in Buddhism, it is Buddha's experience. He experienced certain truths, saw them, came in contact with them, and preached them to the world.
So with the Hindus. In their books the writers, who are called Rishis, or sages, declare they experienced certain truths, and these they preach.
Thus it is clear that all the religions of the world have been built upon that one universal and adamantine foundation of all our knowledge--direct experience.
From Swamiji's book "Raja Yoga," Chapter One, "Introductory." Complete Works, 1.126.
The more we grow in love and virtue and holiness, the more we see love and virtue and holiness outside. All condemnation of others really condemns ourselves.
Retreat given at the Thousand Island Park, USA. July 3, 1895. Complete Works, 7.28.
One day, in the temple-garden at Dakshineswar, Sri Ramakrishna touched me over the heart, and I saw that the houses--rooms, doors, windows, verandahs--the trees, the sun, the moon--all were flying off, shattering to pieces as it were--reduced to atoms and molecules--and ultimately merged in space. Gradually again, the space also vanished and, after that, my consciousness of the ego with it. What happened next, I do not recollect. I was at first frightened. Coming back from that state, again I began to see the houses, doors, windows, verandahs, and other things. On another occasion, I had exactly the same realization by the side of a lake in America.
Conversations recorded in Bengali. From the diary of Sarat Chandra Chakravarty. Complete Works, 5.392.
We are in essence one with God. He who is the coloring in the wings of the butterfly, and the blossoming of the rose-bud, is the power that is in the plant and in the butterfly. He who gives us life is the power within us. Out of his fire comes life, and the direst death is also his power. He whose shadow is death, his shadow is immortality also.
From a lecture on "What Is Religion?". Complete Works, 1.338.