An Isthadev is the Awataar of the Paramaatma a Hindu wishes to follow to attain liberation. It is important for a Hindu to find out whom their Isthadevata is, EVERY HUMAN IS BORN WITH AN ISTHADEVATA, the person just has got to find out which Divine form of the Paramaatma is their Isthadev. The Isthadevata of many devotees are all not the same, to some it is Shri Krishna to another it might be Lord Shiva, others might love Lord Vishnu, some devotee’s Isthadev are Mata Durga, but tt the end of the day we all are worshipping the Paramaatma, and once a devotee knows whom is their Isthadev, they can then look at the scripture they spoke in to see how He guides his devotees to Moksha praapti (Salvation), for example: If Shri Krishna is ones Isthadevata, they should check the SHRIMAD BHAGWAD GEETA to read what he says to Arjuna, If Lord Shiva is ones Isthadevata, they should check the SHIVA MAHAPURAAN to see what Shivji tells to Parwati Mata for the wellbeing for us humans, If Shri Ram is ones Isthadev, they should read the RAMAYANA to see what Gosai Swami Tulasi Das says to attain liberation, If ones Isthadevata is Hanumanji they should also read the RAMAYANA to know the proper procedures to worship Hanumanji, If ones Isthadevata is a form of The Divine Mother Durga they should read, SHIV MAHAPURAN, DEVI BHAGAWAT etc to gain information about how to attain Maa Durga. This is why it is important to know who you are worshipping, and which Divine form of the Paramaatma's way they wish to gain to get to the Lord, at the end, all devotees of Lord Shiva, Lord Vishnu, Shri Krishna, Shri Ram, Hanumanji, Devi Maya, Shree Ganesh etc will attain the Paramaatma, but they still need to know which path they are taking to attain him. The ways one knows whom their Isthadev is: When they hear the name of their Isthadev, tears come to their eyes, the hairs on their arms will stand, they shall continuously want to hear the stories of their Isthadev. These are some signs that this form may be you Isthadevata. Every human being is born with and Isthadevata, but it is their duty to find their Isthadev, if one is starting to find their Isthadevata, it will not happen over night, it will not be like "I have started looking for my Isthadev" and the next day "I have found my Isthadevata". It will take hard work, they should, sing kirtan of the Lord, listen to the different stories of each form of the Paramaatma, and then they should go back and see which form put tears of happiness into their eyes. It is important that one find their Isthadevata, otherwise they will roam around like a fool, not knowing what to do, they will always have problems and at death they will realize that they have made a huge mistake by not trying to find him, not worshipping him, therefore it is important to find you Isthadev. When one finds their Isthadev, they should not impose on other people with another different Isthadevata. For example if ones Isthadev is Shri Krishna and theirs is Lord Shiva, they should respect that they worship Shri Krishna instead of insulting them, in-fact after one has found their Isthadevata, they should promote every-single aspect of SHREE Sanatan Dharma.