The seer of the Isavasya Upanishad prescribes in the first Sloka the method to enter into Jnana Nishtha. The meaning of the Sloka is: "All this, movable and immovable, is indwelt by the Lord. Renounce the names and forms. Renounce the sensual pleasures and enjoy the Atmic bliss. Do not covet anybody's wealth." This is the path of Nivritti Marga for Sannyasins. In the second Sloka he prescribes the Karma Nishtha for householders, who are not competent to follow the path of renunciation by taking Sannyasa.
Kurvanneveha karmani jijeevishechchatam samah
Evam tvayi nanyathetosti na karma lipyate nare.
"Should one wish to live a hundred years on this earth, he should live doing Karma. While thus, as man you live, there is no way other than this by which Karma will not cling to you."
The Narayana Upanishad says: "In the beginning these two roads were laid-the road through Karma and the road through Sannyasa. The latter consists in the renunciation of the threefold desire of wife, son and wealth. Of these, the road through Sannyasa is preferable."
The Taittiriya Upanishad also says: "Renunciation (Nyasa) certainly is to be preferred."
In the Gita also Sri Krishna mentions of Karma Nishtha and Jnana Nishtha:
Lokesmin dvividha nishtha pura prokta mayaanagha
Jnanayogena sankhyaanaam karmayogena yoginaam.
"In this world there is a twofold path, as I have before said, O sinless one: that of Yoga by knowledge of the Sankhyas, and that of Yoga by action of the Yogis."
Here Sankhya Yoga signifies Vedanta and not the philosophy of Kapila Muni.
Bhagavan Vyasa, after much discussion told his son his firm conviction: "These then are the two roads on which the Vedas are based. Both the courses-one which leads to Karma and the other which draws away from Karma-have been explained, etc."
Nishtha means perfect devotion and sticking to one's ideal. There is no wavering or oscillation of the mind here. The Yogi of Karma Nishtha has the firm, unshakeable conviction that his Nishtha will lead him to the goal and attainment of supreme bliss. He plunges himself in Karma. He is absorbed in Karma. Raja Janaka had his Nishtha in Karma Yoga. Sri Mahatma Gandhiji was a Yogi of Karma Nishtha. He never deviated from his ideal. He was established in his Nishtha. Nishtha is a sine qua non for success. Then only one can apply his full heart, mind and intellect to the work on hand. Failure is due to lack of Nishtha. Nishtha develops will-power. Nishtha helps quick growth and rapid evolution.
Nishtha removes the stumbling blocks on the path of realisation.