Bhagavad-Gita is one of the holy books of Hindus and worlds largest reading book. And it is a part of Mahavarata another Holy book of Hindus.
On it Lord Krishna makes a speech to Arjuna where Arjuna is in a religious war and fought against the iniquity of Kauravs who was their cousin-brothers. And when Arjun felt guilty to fight against their relatives.
Then Lord Krishna convinced Arjuna to fight for the truth. And give him(the human beings) advice.
Whatever has happened has happened for good.
(যা হয়েছে তা ভালই হয়েছে,)
Whatever is happening is also for good.
(যা হচ্ছে তা ভালই হচ্ছে,)
Whatever will happen, shall also be good.
(যা হবে তাও ভালই হবে।)What have you lost that are you cry for?
(তোমার কি হারিয়েছে, যে তুমি কাঁদছ ?)What did you bring, that you have lost?
(তুমি কি নিয়ে এসেছিলে, যা তুমি হারিয়েছ?)What did you create, that was destroyed?
(তুমি কি সৃষ্টি করেছ, যা নষ্ট হয়ে গেছে?)What you have taken, has taken from here
(তুমি যা নিয়েছ, এখান থেকেই নিয়েছ।)What you gave has been given here.
(যা দিয়েছ এখানেই দিয়েছ।)What belongs to you today,
(তোমার আজ যা আছে)Belonged to someone yesterday.
(কাল তা অন্যকারো ছিল)And will be someone else's tomorrow
(পরশু সেটা অন্যকারো হয়ে যাবে।)Change is the law of the universe.
(পরিবর্তনই সংসার এর নিয়ম।।)
Writing on mythology and its best choice is Bhagavad Gita"
source: Bhagavad gita As It Is
for your interest, if you want to read: Increase your power of yoga- How Lord Krishna has said in Bhagabat Gita