Lord Krishna manifest on mother earth above 3650 years before. It was in the battle of Mahabharata between Kauravas and Pandavas the sermon of Bhagavad Gita was delivered by Lord Krishna to Arjuna.
Many years have passed since then but nothing in Bhagavad Gita diluted with time. Everything contained in Bhagavad Gita is 100% pure. Nothing in this world paralleled Bhagavad Gita.
The sermon of Bhagavad Gita by Lord Krishna to Arjuna was necessitated as Arjuna expressed cowardice when facing the enemy head-on. The reason given by Arjuna seemed correct. He did not want to fight his cousins, his guru Dronacharya even maternal grandfather Bhishma Pitamah and many other close relatives and friends.
Lord Krishna not only admonished Arjuna but also emphasized on him that war of Mahabharata was no ordinary fight, it was a battle to re-establish Dharma (righteousness) on mother earth. Evil had to be obliterated under all circumstances and costs. If his maternal grandfather, guru even close relatives and friends sided the enemy, the evil it was their problem, not his.
After much persuasion that Arjuna represented more than 1 million countrymen and forces, Arjuna prepared himself to fight.
The fight of Mahabharata represents an ongoing war between positive and negative forces of nature all the time. This battle is ensuing in all human beings every single moment of life. Sometimes the good wins and sometimes the bad! In both the circumstances remaining neutral human beings must try to overpower their negative tendencies forever.
Difficult yet not impossible!
Some say Bhagavad Gita is not such an old epic, others say the battle of Mahabharata never happened, some even go to the extent that Lord Krishna never existed but whatever the truth; the philosophy contained in Bhagavad Gita could never be falsified.
Bhagavad Gita is equally applicable today as in olden times. It is the real story of human life, the story of a war, battle that goes on inside all human beings all the time.
Not falling into jargon of history, we must try to realize the potential of Bhagavad Gita by travelling path of spirituality. Existence or no existence, the doctrine of Bhagavad Gita forms summum bonum of spirituality following which human beings succeeded becoming a Mahavira, Gautama Buddha, Jesus Christ and Prophet Mohammed in their lifetime.
Whether Bhagavad Gita is a story, a doctrine propounded by Veda Vyasa or it really happened does not matter, what matters is following path of spirituality to liberate self from cycle of birth and death forever.
I indulged in path of spirituality from six years of age. Not for a single moment ever in my life I doubted existence of Lord Krishna and his sermon of Bhagavad Gita, prime reason why I succeeded in reaching state of self realization, god realization 37 years of age.
Human beings are born wise, we have been endowed with an independent will, power of discrimination exercising which we can easily undertake spiritual journey to reach god.