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Gandhi believed that his non-violent resistance was a way that was blessed by Truth, which is powerful and active. Non-violent resistance works, he believed, not just because of its effectiveness as a means to persuade others and draw them to your cause, but because Truth is living and powerful and would work on behalf of the non-violent who stood up for Truth.
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Gandhi would have the concept of Brahman, that is quite odd for Westerns. Brahman is understood as the Absolute Truth, Ultimate Reality, Supreme Being, not actually like the Western God who is a dual god and may control one's karma or activities by giving one punishment and reward for his deeds.
Brahman is considered as unbiased and it won't deal with dual conditions such as pious and sinful activities. After final release one may attain the position of Brahman where there are not dualities such as sin and merit.
Sinful and meritorious deeds are not hindrance or the cause of final release, but only vidya (self-knowledge). So, Brahman is attained only by that vidya, that is the cause of liberation (moksha).
According to Advaita-vada, that is the main Philosophic line from Hinduism, ahimsa (non-violence) and himsa (violence) are harmonized only by Brahman himself and should not be taken as merit or sin.
Bhagavad-gita, that is the main scripture followed by most of Hindus, explains that Arjuna has opted for ahimsa (non-violence) and Krishna has showed Arjuna that the best way was actually himsa (violence). Repeating the dialog between Krishna and Arjuna, at the end violence has prevailed, as Gandhi himself had to face the violence of India's independence war among Hindus and Muslins and his own murder. Thus, the result of ahimsa was only extreme violence like the lesson given by Gita.
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I believe there is not just God, but Truth, and that Truth was put on earth by God. Those who love justice and show mercy will find it effective and powerful to behave in such a way, because they belong to Truth and Truth supports those who walk in his ways, even if they're atheists.
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According to Advaita-vada, Brahman, or the Asolute Truth is not a religion god who is in charge of mundane creation, as there are different levels of creation (sarga and visarga). Brahman makes sarga (original forms and ideas), and the mundane creation (visarga) is made by Brahma who is an ordinary soul.
Brahman won't be influenced by one's love and hate, justice and injustice, mercy and punishment as Brahman is taken as a non-dual substance and perfectly unbiased. Brahman does not love anyone and Brahman does not hate anyone, it has no friends and it has no foes at all. He is equal for everyone.
Theism and atheism are taken as mere inferences that cannot be helpful to attain vidya (self-knowledge) and soul's final release.