Sri Chinmoy: The only way you can show the existence of God to that kind of person is by your own personal inner peace, inner light and inner joy. You can show the Supreme in any of the infinite aspects or infinite ways in which He manifests Himself. You do not have to show Him in a human form, even if you were able to do this. Since the Supreme is infinite Joy, Peace, Light, Bliss and Power, you can reveal Him just by embodying these qualities in yourself. Your very appearance, your very presence, when you stand in front of that person, can make him feel the existence of God inside you.
When you are in the presence of a spiritual person, what do you feel? You feel peace, you feel light. His very presence makes you feel that there is something divine in him. What are you experiencing? It is actually the Supreme in the form of the aspirant's divine qualities. So the best way to demonstrate the existence of God is to manifest God in your own life, in your own consciousness.
You cannot convince others by using words, by endless talking and arguing. Every intellectual argument showing the existence of God can be met by a counter-argument. You will discover this if you study Western philosophy. There is no end to the statements and rebuttals about the existence or non-existence of God. These fruitless arguments have been going on for centuries. Whole philosophical systems have been built up around these intellectual theories. But even if your mind is illumined with your soul's light, that is not enough to convince someone who does not agree with you. The mind of the other person also has to be in touch with his own soul's truth to some extent. Then only will he be able to receive the inner truth of what you say. Needless to say, this rarely happens.
The best way to convince someone of the existence of God is to try to manifest His Divinity in your own life. If the other person is at all sincere or receptive, he will see that you have something that he lacks. He will also feel that your qualities of light and peace are coming from another source. He knows that you are leading a spiritual life. He knows that you believe in God. He will see and feel that you have got inner peace, poise, light and joy. If he is sincere, this is all the proof he needs. His inner perception of a better, higher consciousness in you is the only thing that will convince him. It will convince him more than thousands of words.