Bhagavad Gita is a spiritual treatise, a sermon delivered by Lord Krishna to Arjuna in the battlefield of Mahabharata. Philosophically, metaphorically no sermon was delivered.
From the beginning of life Arjuna was a spiritual traveler undertaking inner journey all the time. Arjuna was interested in reaching God in his lifetime. It so happened that before starting war with his arch enemies Kauravas Arjuna in the battlefield automatically reached state of enlightenment. And in those rare moments he realized shallowness of his thought process.
Finally realizing that he was immortal, a spiritual traveler undertaking earthly journey and not a manifest physical human form people called Arjuna, he prepared to fight the enemy head-on. Arjuna reached peak of his spiritual career just before the start of battle of Mahabharata (a war of dharma, righteousness).
The sermon of Bhagavad Gita delivers a single message is to mankind that we are primarily a spirit, a cosmic traveler on its cosmic journey. This spirit, consciousness manifested human form to work out its karma, remove dross impurities within. Our significance as a body was minimal did not carry much meaning in the spiritual world.
Arjuna realized that human body was only a medium for spirit within to realize itself, become a pure soul atman again. In the battlefield when facing the enemy Arjuna suddenly reached end of cosmic life, became an enlightened one, a bodhi, a Jina. And in those moments his soul atman, Lord Krishna existing within, started dictating from within in clear terms.
Dogged by a bloated ego human beings never wanted to hear the voice of Lord Krishna coming from within. It is only when human beings demolished their ego we could distinctly hear promptings of Lord Krishna coming from within. By indulging in spirituality detailed in Bhagavad Gita any human being world over could easily reach this state of enlightenment.
Mahavira, Gautama Buddha did this followed by Jesus Christ and Prophet Mohammed.
Human life was all about realizing cosmic truths of life. This we can achieve by taking a spiritual journey deep within into the recesses of our heart wherein existed Lord Krishna in all his glory.