I don't wish to debate the idea of making Gita India's national book. For one thing I don't understand what that status will imply. But surely I wish to tell you why I think the Gita is India's most important scripture.
To begin with, for a scripture to be of timeless importance, it has to have direct relevance to our lives. It has to address both ethical and social issues. It has to provide guidance for a seeker of truth. You may give the status of truth to what is scientifically validated, or spiritually experiential, or both.
Second, the scripture has to be easy to read and understand. It has to have layers of meaning which are consistent and that make sense to different readers. It has to talk not through myths and legends but through a more direct discourse.
Third, it should have an authentic text which prevents people from disputing endlessly for versions and interpretations.
Fourth, it should have a transformational power which rules the minds and hearts of men.
The Gita qualifies on every one of these criteria, as will be seen from a study of the work and its various commentaries. Let me add some more points.
First of all it is a direct dialogue between Krishna and Arjuna, a warrior, standing ready to battle his enemies for his right to the kingdom, which has been taken away wrongly. His friend and guide Krishna has become his charioteer. Arjuna has sudden doubts of what's right for him to do. He gets advice from Krishna. For those confused about Gita's context for the righteous war, I suggest reading the Mahabharata first. And then Gita. Anyone who has not read either Mahabharata or the Gita seriously has no right to take part in this discussion.
Krishna gives his advice first at the level of responsibility for a warrior readied for battle for a cause he has stood for, all his life. This gives a social and ethical basis for discussing one's duty within the context of Arjuna's life situation.
But Gita quickly moves into ideas of the nature of existence and creation, how man acquires his tendencies and propensities, how he has to deal with conflict, how he has to give back to life. We may feel uncomfortable with Hindu concepts of rebirth, karma and such. But we are discussing here a scripture of this nation, and these ideas are implicit to our way of life and form the fabric of thinking for an entire civilisation. That is why both Buddha and Mahavira accepted rebirth and karma theory as the basis of their teachings also.
The concept of karma and rebirth provides a strong basis for ethics.
Now Gita does not stress image worship, ritualistic prayer and ostentatious ceremony. Very briefly, Krishna says that all duty done well is a sacrifice or offering that helps the entire circle of life. Worship is more important in spirit and attitude. Worship is not meant to be ostentatious display. In fact Krishna says it is evil and unethical to horde and display wealth. Wealth is to be treated only as a trust.
There are far reaching discussions also in the Gita about-
Discipline in habits, day-to-day living,
Moderation in rest and recreation,
Leading by example others- especially those who look up to a man of influence,
Taking failure as a stepping stone to success,
Treating success with equanimity,
Giving to others generously,
Speaking the truth,
Contemplation and solitude for self-development,
Becoming attuned to the circle of life.
Discipline in habits, day-to-day living,
Moderation in rest and recreation,
Leading by example others- especially those who look up to a man of influence,
Taking failure as a stepping stone to success,
Treating success with equanimity,
Giving to others generously,
Speaking the truth,
Contemplation and solitude for self-development,
Becoming attuned to the circle of life.
All these ideas as developed and discussed in the Gita have no mythological dimensions but are dipped in immense practicality.
Man has also to find answers and meanings in life as well as in death. Krishna deals with these aspects in a most inspiring and thought- provoking manner.
Finally Krishna also lists what's wrong- meaningless ritual, blind adherence to practice without commitment to the larger goal of life, and a dogmatic response to reality.
I don't know of any other such timeless classic in Indian civilisation. Bhagavad Gita is for me the scripture for the Hindu way of life.