Bhagavad Gita

ಸುಖದುಃಖೇ ಸಮೇ ಕೃತ್ವಾ ಲಾಭಾಲಾಭೌ ಜಯಾಜಯೌ ।
ತತೋ ಯುದ್ಧಾಯ ಯುಜ್ಯಸ್ವ ನೈವಂ ಪಾಪಮವಾಪ್ಸ್ಯಸಿ ॥೩೮॥
sukhaduḥkhe same kṛtvā lābhālābhau jayājayau |
tato yuddhāya yujyasva naivaṃ pāpamavāpsyasi ||38||

Gist of the sloka:
Fight only after viewing happiness & sadness, win and lose, earning & losses in the same manner – for it will not earn you any papha.
Happiness & sadness, win and lose, earning & losses are all inevitable in one’s life. If one perceives these up’s and down’s in a calm mind, then there is no accumulation of sins. One should therefore perform his actions as duties, that is the secret of success.
Lord Krishna says: One should without any selfish aim to win and accumulate riches, without desire that my opponents should face defeat/loss, with the sole aim that everything that happening is as per Lord; and one only need to do his duty and fight the war.
If all others also follow the steps given by the Lord in their lives, one will never face grief. One should follow the principle: whatever comes, let it come [we will face it]; but Lord give me Your grace at all times.
One who always adheres to the dharmic path, the doors of the heaven is always open for him. He will not see grief as grief, loss as a loss; for he will see heaven here and above.
So far Lord described the nature of soul, what living means, what is death, difference between soul and Lord [paramatma], what is the secret behind all this, what is in existence for ever etc., He also described the necessity of the war. So far the verses described the science behind [Eshwara or Belief in God] Shaankya shastra [science]. There is another kind of [Nir Eeshwarya] Shaankya which does not belief in existence of God. Lord does not reference it anytime.
It’s our practical experience that whenever we read/hear such spiritual books like Gita, our mind temporarily feels great and we even wish to remember it forever. In reality, we forget it, moments later. So is the case with Arjuna. He after getting the lessons directly from the Lord himself, forgets everything [in a few days’ time] and laments due to his son’s Abhimanyu’s death. To obtain the right knowledge and experience to achieve mukthi is no easy task. By just understanding that soul/atma is immortal is not enough. We need to strive to understand the entire science behind it. Once a person understands it completely, then grief and happiness means the same.
Question would be how do we understand the complete knowledge about Lord and souls? What is the means to do so? How do we retain such a knowledge permanently? Lord Krishna takes us now on that journey, answering all of the above.