The notion of God means an absolutely perfect being. An absolutely perfect being must have all the positive attributes, including the attributes of existence. So God must exist.
The existence of God cannot be proved by scientific experimentation. It is purely a question of faith and refers to the intuitive side of man.
The deepest craving, the deepest aspiration in man is for eternal happiness, eternal knowledge and eternal Truth. Man should search for some supernatural entity which can satisfy his deepest craving and aspirations.
As we explain everything within Nature by the law of cause and effect, so also Nature as a whole must be explained. It must have some cause. This cause must be different from the effect. It must be some supernatural entity, i.e., God.
Nature is not a mere jumble of accidents, but an orderly affair. The planets move regularly in their orbits, seeds grow into trees regularly, the seasons succeed each other in order. Now Nature cannot order itself. It requires the existence of an intelligent being, i.e. God, who is responsible for it. Even Einstein, the great scientist, was strongly convinced of the creation of the universe by a Supreme Intelligence (Click Here for some of Einstein's quotes on spirituality).
Everything in Nature has some purpose. It fulfills some function or other. Certainly every object by itself cannot choose a function for itself. Their different functions ought to have been planned or designated by a single intelligent being or God.
Albeit everything is transitory in this world, people purchase enormous plots of land, build bungalows in various places and erect five-storeyed houses. They want to establish eternal life in this sense-universe. This shows that man is essentially immortal. In spite of the knowledge that everyone has to die, man thinks that he will live for ever and make very grand arrangements to live here perpetually. Further, nobody wants death. Everybody wants to live, and takes treatment when ill, spending any amount. Hence, the essential nature of man should be eternal existence.
Even a fool thinks that he is wise. Everyone wants to show that he knows more than others. Nobody likes to be called a fool. Children tease their parents with various sorts of questions. The desire to know is ingrained in them. These indicate that our essential nature is knowledge.
When a man laughs, people seldom ask why he is laughing. On the other hand when a man cries, everybody asks why he is crying. This shows that our essential nature is bliss. No one wants misery, but everyone want happiness, and all one's activities in life are directed towards the acquisition of happiness. This also proves that our real nature is bliss. In deep sleep when there are no objects, senses or mind, we feel bliss; hence our essential nature should be bliss. That again is the reason why people ailing from painful maladies even desire to give up their bodies and thus get rid of the pain.
If everyone then is of the nature of existence-knowledge-bliss, there should be an all-pervading principle having these characteristics and different from the perishable, inert pain-giving physical bodies. Therefore, Brahman or God, whose nature is Existence-Knowledge-Bliss (Sat-Chit-Ananda), should exist.
The existence of God or the Self is determined or indicated by the existence of the Upadhis or limiting adjuncts, viz., body, mind, Prana and the senses, because there must be self-consciousness behind their activities.
You always feel that, despite your possessions and all sorts of comforts, you are in want of something. There is no sense of fullness. Only if you add to yourself the all-full God, you will have fullness.
When you do an evil action, you are afraid. Your conscience pricks you. This also proves that God exists and witnesses all your thoughts and actions.
To define Brahman is to deny Brahman. The only adequate description of Brahman is a series of negatives. That is the reason why the sage Yajnavalkya declares in the 'Brihadaranyaka Upanishad' about Brahman as neti, neti, or 'not this', 'not this.' This means that the residue left after sublating the names and forms is Brahman.
Brahman or the Self or the Immanent God cannot be demonstrated as He is beyond the reach of the senses and mind but His existence can be inferred by certain empirical facts or common experiences in daily life.
Sometimes you are in a peculiar dilemma or pressing pecuniary difficulty. Help comes to you in a mysterious manner. You get the money just in time. Most of you might have experienced this. You exclaim at that moment in joy "God's ways are mysterious indeed; I have got now full faith in God. Up to this time I had no faith in God." An advocate had no faith in God. He developed double pneumonia. His breath stopped. His wife, son and relatives began to weep. But he had a mysterious experience. The messengers of Yama caught hold of him and brought him to the court of Lord Yama. Lord Yama said to his messengers: "This is not the man I wanted. You have brought a wrong person. Send him off." He began to breathe after some time. He actually experienced that he left the body, went to the court of Yama and again re-entered his physical body. This astonishing experience changed his entire nature. He developed an intense faith in God and became a religious man.
Another educated person had a similar experience, but there was some change in this case. He was also an atheist. His soul was brought by the messengers of Yama to his court. This person asked Yama: "I have not finished my work in the physical plane. I have to do still more useful work. Kindly spare my life now." His boon was granted. He was struck with wonder on this strange experience. His nature also was entirely changed. He left his job at once. He devoted the remaining portion of his life in selfless service and meditation.
You find that even the world's best doctors fail to cure a dying king. You might have also heard of many instances where patients ailing from the worst type of diseases are cured miraculously where even the ablest doctors have declared the cases hopeless. This itself is a clear proof that there is the divine hand behind all cures.
Sometimes you cannot tolerate the company of persons. You wish to remain alone. You go to a solitary place, - in a garden or in the banks of a river - and enjoy the inner peace. This gives the clue that you are, in essence, an embodiment of peace, that you are alone and identical with Brahman.
Some people die when they are eighty years old; some die when they are in the womb; some die at twenty; some at forty. What is the cause for the variation? Who has fixed the span of life for all? This clearly proves that there is the theory of Karma, that there is one Omniscient Lord who is the dispenser of the fruits if the actions of the Jivas (individual souls), who fixes the span of life of the Jivas in accordance with their nature of Karmas or actions, who knows the exact relation between Karmas and their fruits. As Karma is Jada or insentient, it cannot dispense with the fruits of their actions.
Whether the owl accepts the presence of light or not, there is always light. Whether you accept the existence of God or not, He always exists. He is ever shining in the 'three periods of time.' He exists before you begin to search for Him. He is closer to you than your breath and nearer to you than your hands and feet.
Do you exist or not? This is my question. If you say 'No', I see before my eyes your figure with sinewy arms, a broad forehead and big eyes. If you say, 'Yes', this gives the clue to prove the existence of God. The very question whether God exists or not clearly proves that God exists.
Whatever you see is God. Whatever you hear is God. Whatever you taste is God. Whatever you smell is God. Whatever you feel is God. This is the manifested aspect. The physical body belongs to the Virat (Cosmos). The astral body belongs to Hiranyagarbha (Cosmic intelligence). The causal body belongs to Ishvara (Reality with its veiling power). Where is this 'I' now?
Emerson says: "A little consideration of what takes place around us every day would show us that a higher law than that of our will, regulates events: that our painful labours are very unnecessary and altogether fruitless: that only in our easy, simple, spontaneous action are we strong, and by contending ourselves with obedience we become divine. Belief and love will relieve us of a vast load of care. O my brothers! God exists. There is a soul at the centre of nature and over the will of every man, so that none of us can wrong the universe. It has so infused its strong enchantment into nature that we prosper when we accept its advice; and when we struggle to wound its creatures, our hands are glued to our sites, or they beat our own breasts. The whole course of things goes to teach us faith."
Can you tell me, friends: Is there any one on the face of the earth who is not afraid of death? Is there any one who is not uttering the name of the Lord when he is in serious difficulty, when his life is trembling in the balance or when he is in acute agony? Why then do you deny the existence of God? You yourself admit His existence when you are in trouble. On account of perverted intellect and worldly intoxication you have turned out to be an atheist, though to be sure, you cannot prove His non-existence by any means. Is this not a great folly? Think seriously. Give up arguing. Remember Him and attain immortality and eternal peace.