Worship of one god and worship of Shri Ram
Worship others is not unauthorize. It is a big system and other gods are also part of the system.
There is a word "Ramsification" means subdivisions of a big system.
Lord Vishnu holds Rajas nature and he knows how to tackle Arjuna in Mahabharata so he told him that only worship me.
Lord Vishnu also knew other people's karma and also the fate of the war also the end of the war. But to execute it through the soul of Arjuna he guided him to give his chariot in the hands of Lord Krishna.
Any Hindu chant or invocation starts with
Aum which means I bow to all Gods and I salute them.
Then the name of the lord is chanted like Om namah shivay, Om Ramay namah or Om namo bhagawate Vasudevay or Om Krishnay namah.
Other than this Shree is the starting point.
Shree Krushna Sharanam mama Shree Shivay Namah Shree Ramay Namah
Shree means Auspicious. Means do auspicious to me. "Shubham kuru".
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