"Bhagavad-gita" is the main book for all humanity because the message of "Bhagavad-gita" does not have any borders. Now people all over the world read this book and change their lives according to it. To read "Bhagavad-gita" there is no need to change religion, by reading it some people better understand the Bible. "Bhagavad-gita" is spiritual science, and science is universal.
We should follow "Bhagavad-gita" and take it seriously, because this is the most urgent message which all humanity should know. Especially now when the whole world is in crisis. People are trying to offer some solutions, but we can not solve a problem with the same mind which created a problem, so we need to change the mind.
The main message of "Bhagavad-gita" is to remember Krisna, to become His devotee and go back to Krisna. We can remember Krisna by chanting Maha-mantra. The sound of Maha-mantra is not of this world, it helps us to connect with God. Our relationship with God is eternal, if we remember Him at the end of life, we will return to Him. To remember Krisna at the end of life we should remember Him now.
Srila Prabhupada explained the message of "Bhagavad-gita" in such a powerful and beautiful way that a lot of people in this world take this book as the main book in their life. If we take the message of "Bhagavad-gita" very seriously, we will become very happy as a result.