What is the greatest religion and the greatest action for a human being?

Srimad Bhagavatam Mahapurana ( 1.2.5-11)
The story of the Bhagwat and the importance of devotion to the Lord

The best religion for human beings is that by which there is devotion to the Lord such that there is no desire of any kind and which remains eternally continuous.

As soon as there is devotion to Lord Krishna the mind is connected to him with love, a devotee gets pure knowledge and renunciation.

Even after properly performing Dharma, if there is no passion for the stories of the Lord's pastimes in the heart of a man, it is mere labor.

The fruit of righteousness is salvation. Its significance lies not in the attainment of materialistic achievements. Pleasure is not supposed to be its fruit.

The fruit of pleasure is not to satisfy the senses, its purpose is only to maintain life. The fruit of life is also elemental curiosity. Getting heaven and other things by doing a lot of karma is not his fruit.

The philosophers call the undivided, unique, and absolute consciousness (Saccidananda) that is devoid of the distinction between the knower and the known, as Tattva. Some call it Brahman, some call it Paramatma, and some call it God.