( Story told by Swami Vivekananda )
Take up one idea, your Ishta, and let the whole soul be devoted to it. Practise this from day to day until you see the result, until the soul grows. And if it is sincere and good, that very idea will spread till it covers the whole universe. Let it spread by itself; it will all come from the inside out. Then you will say that your Ishta is everywhere and that He is in everything.
Of course, at the same time, we must always remember that we must recognize the Ishtas of others and respect them—the other ideas of God—or else worship will degenerate into fanaticism. There is an old story of a man who was a worshipper of Shiva. There are sects in our country who worship God as Shiva, and others who worship Him as Vishnu. This man was a great worshipper of Shiva, and to that he added a tremendous hatred for all worshippers of Vishnu and would not hear the name of Vishnu pronounced. There are a great number of worshippers of Vishnu in India, and he could not avoid hearing the name. So he bored two holes in his ears and tied two little bells onto them. Whenever a man mentioned the name of Vishnu, he moved his head and rang the bells, and that prevented his hearing the name.
But Shiva told him in a dream, ‘What a fool you are! I am Vishnu, and I am Shiva; they are not different—only in name. There are not two Gods.’ But this man said, ‘I don’t care. I will have nothing to do with this Vishnu business.’ He had a little statue of Shiva and made it very nice, built an altar for it. One day he bought some beautiful incense and went home to light some of the incense for his God. While the fumes [smoke] of his incense were rising in the air, he found that the image was divided into two: one half remained Shiva, and the other half was Vishnu. Then the man jumped up and put his finger under the nostril of Vishnu so that not a particle of the smell could get there.
Then Shiva became disgusted, and the man became [was turned into] a demon. He is [known as] the father of all fanatics, the ‘bell eared’ demon. He is respected by the boys of India, and they worship him. It is a very peculiar kind of worship. They make a clay image and worship him with all sorts of horrible smelling flowers. There are some flowers in the forests of India which have a most pestilential smell. They worship him with these and then take big sticks and beat the image. He [the ‘bell eared’ demon] is the father of all fanatics who hate all other gods except their own.
Source: The Complete Works of Swami Vivekananda/Volume 9/Lectures and Discourses/Bhakti-Yoga