Extracted from Swami Vivekananda’s "Bhakti Yoga":
The next points to be considered are the worship of Pratikas or of things more or less satisfactory as substitutes for God, and the worship of Pratimâs or images.
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Now worshipping Ishvara and Him alone is Bhakti; the worship of anything else — Deva, or Pitri, or any other being — cannot be Bhakti. The various kinds of worship of the various Devas are all to be included in ritualistic Karma, which gives to the worshipper only a particular result in the form of some celestial enjoyment, but can neither give rise to Bhakti nor lead to Mukti [...]
[When you are worshipping Pratikas-images] if the image stands for a god or a saint, the worship is not the result of Bhakti, and does not lead lo liberation; but if it stands for the one God, the worship thereof will bring both Bhakti and Mukti. Of the principal religions of the world we see Vedantism, Buddhism, and certain forms of Christianity freely using images; only two religions, Mohammedanism and Protestantism, refuse such help. Yet the Mohammedans use the grave of their saints and martyrs almost in the place of images; and the Protestants, in rejecting all concrete helps to religion, are drifting away every year farther and farther from spirituality till at present there is scarcely any difference between the advanced Protestants and the followers of August Comte, or agnostics who preach ethics alone. Again, in Christianity and Mohammedanism whatever exists of image worship is made to fall under that category in which the Pratika or the Pratima is worshipped in itself, but not as a “help to the vision” (Drishtisaukaryam) of God; therefore it is at best only of the nature of ritualistic Karmas and cannot produce either Bhakti or Mukti. In this form of image–worship, the allegiance of the soul is given to other things than Ishvara, and, therefore, such use of images, or graves, or temples, or tombs, is real idolatry; it is in itself neither sinful nor wicked —it is a rite — a Karma, and worshippers must and will get the fruit thereof.