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So if according to Advaita Ishwara is formless and can assume any form which the devotee desires, are some forms holier than others, what is the difference between the forms of Lord Vishnu or Lord Shiva and say a Greek God Zeus or some tribal God or a celebrity film star. Why can't someone worship the form of God Zeus or Lord Indra or some random celebrity as Ishwara? Like is there a preference for Upasya Roopa according to orthodox Advaita?

If all forms of Ishwara is equally valid, then, how are, holy Hindu forms of Lord Vishnu and Lord Shiva, more special and holy than other forms of Greek and Roman gods or popular celebrities?