The Bhagavad-Gita, a sacred Hindu text, was a philosophical discussion where Krishna instructs the prince, Arjuna, in principled matters and the nature of God. The Hindus believe in succeeding in completing your dharma, to get good karma, to further advance your atman. This is the point of the entire Bhagavad-Gita. .
The Bhagavad-Gita told Hindus everything they needed to know about the religion, by trying to get the same points across. For instance, verse 31 in the 11th teaching states, "I exist in all creatures, so the disciplined mad devoted to me, grasps the oneness of life; wherever he is, he is in me." This rather unclearly explains that one's atman is something that in no way can be destroyed. Your atman is you. One's atman goes through several different lifetimes, trying to reach the up-most point in the reincarnation cycle.
Verse 23, the 12th teaching, "Weapons do not cut it, fire does not burn it, waters do not wet it, wind does not wither it." This verse also explains that one's atman cannot, in any way, be harmed, and/or destroyed. However, one's atman CAN be changed. Over the course of several lifetimes, one's atman's karma is increased. The better you do in achieving your dharma, the better karma you will have. Nothing can harm your atman, in a physical sense. Technically speaking, if you do not achieve your dharma, or if you cause horrible things in any of one's atman's lifetimes, one's atman could be "harmed", in the sense that they will change for the worse.
In conclusion, the Bhagavad-Gita made up most of the principles of the Hindu religion (setting guidelines, rules, ect.). The Bhagavad-Gita explains to Hindus that you need to achieve your dharma, to get good karma, to improve your atman, which will carry you into a better afterlife. The Hindu religion was based upon the principles set by the Bhagavad-Gita, and yet the Bhagavad-Gita itself was meant to be only a guideline.