A Hindu who never worships any god except Shiva, but doesn’t object to his neighbour’s worshipping Krishna or Durga, fails the test of monotheism.
A Shiva worshiper does not object to a Krishna or Durga worshipper, because he understands they are worshiping the same god. Most Hindus you talk to will tell you there is only one god, but infinite are his forms. This is hardly just a modern Neo-Vedanta tack on or a response to Christian monotheism, it is as old as the Vedas, “There is only one supreme truth, with many names like Indra, Varuna, Aryaman” “There is only one without a second, not two or three, four or five, six or seven” “In the beginning there was only one, and nothing else”
In classical Vedanta of the Upanishads this ONE is called Brahman. The supreme, infinite, absolute reality. There can only be one infinite. Multiplicity is explained as Brahman appearing in infinite forms in reality through the power of maya. Hence the concept of Nirguna Brahman and Sadguna Brahman; Brahman without attributes and qualities and Brahman conceptualized by humans as having attributes and qualities(Brahman as the divine architect, Vishvakarma; Brahman as the fire divine, Agni; Brahman as the supreme lord, Indra; Brahman as the vital force, Vayu) Vedanta enjoins one to directly worship, meditate and contemplate on Nirguna Brahman. In the Chandogya Upanishad when a sage approaches King Janaka with instructions on meditation, the sage tells him to meditate on various forms such as light, wind etc, King Janaka keeps falsifying him, “No, one should not meditate on light, but should meditate on the principle of luminosity, the supreme light of Brahman” Whatever Sadguna Brahman the sage tells, Janaka falsifies it with the Nirguna Brahman.
In principle it is recognized in Hinduism that worshipping Sadguna Brahman is considered an inferior practice(Swami Vivekananda, Sri Aurobindo echo the same) because effectively you are worshiping only human imagination. Even the Agamas recognize this is an inferior practice, brought on by Kaliyuga, where people’s intellect is too unrefined to work with Nirguna Brahman. Guru Nanak reformed Hinduism to form Sikhism by getting rid of Sadguna Brahman worship and replacing it with Nirguna Brahman. This is why in the famous Mool Mantra of the Sikhs we find:
Ek Onkar: There is only supreme reality
Sat Naam: He is the true name
Karta Purka: He is the supreme creator
Nir Bahu: He is fearless
Nir vair: He is without discrimination
Akal Murat: He is absolutely formless and timeless
Ajuni: He is beyond birth and death
Sai Bhang: He is self-existent
Gur Purshad: He is realized by the true guru’s grace
Jap: Mediate on his name
Aad Sach: He was true in the timeless beginning
Jagad Sach: He is true through the ages
Hai Bhi Sach: He is true now
Nanak Hosi Bhi Sack: Nanak says, he will always be true
All practicing Sikhs chant this mantra everyday. It is thus clear that Sikhism does not at all endorse idolatry. It is fundamentally opposed to the spirit of it. Very much like classical Vedanta of the Upanishads or Advaita of Sankara is. The practice of creating Sadguna Brahman image is considered completely unnecessary and the practice of the lowest denominator of spiritual seekers, who worship human imagination, mythical creatures. It really is not necessary, because all you need to do is simply remember god(Naam Simran) in your heart. Sit quietly with yourself and dedicate yourself to god. You do not need to go to any temple or erect statues and worship them every day - none of this fanfare and grandiosity is required, moreover it is distracting. Just quietly go within yourself. There god resides.

