A saint is a man or a woman who has renounced worldly life and taken up the path of liberation and attempts to get as close to the divine as possible. He/she is the one who is free from egotism, my-ness and duality. Saints are usually unselfish, practice unconditional love for all, and are endowed with equal vision. Saints are beyond dualities like likes and dislikes, bad and good, profit and loss, pleasure and pain, and heat and cold. They are devoid of greed, anger and lust. They are embodiment of Divine knowledge.
Hindu Saints are from many walks of life, they are called Gurus, Sadhus, Rishis, Swamis and other names as well. However, saints are not apostles.
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Other reading material:
Relevant videos: Swami Nikhilanand Hinduism Q & A: What Is A Saint
Contributor: Geetha Ravula