Formerly, the varnasrama-dharma was prominent, and everyone had a particular duty according to his position in society. Now the occupational duties have expanded, but it doesn’t matter whether one is an engineer, a doctor or whatever. Simply try to serve Kṛṣṇa by the results of work. That is bhakti. It is not the philosophy of this Kṛṣṇa consciousness movement to disengage people from their activities. One should engage in his occupation, but one should never forget Kṛṣṇa. Kṛṣṇa advises us to always become Kṛṣṇa conscious, and we should always think that we are acting for Kṛṣṇa. Of course, we should work by the order of Kṛṣṇa or His representative, not whimsically. If we perform some nonsensical action and think, “I am doing this for Kṛṣṇa,” that will not be accepted. The work must be verified by Kṛṣṇa’s acceptance or by the acceptance of Kṛṣṇa’s representative.
If we conduct business and earn money, we should spend it for Kṛṣṇa. This is a form of bhakti. Another vivid example is Arjuna, who was a fighter. By fighting, he became a devotee. He did not become a devotee by chanting Hare Kṛṣṇa but by fighting in the Battle of Kuruksetra. Kṛṣṇa advised him to fight, but because Arjuna was a Vaisnava, in the beginning he was unwilling. A Vaisnava does not like to kill anything, but if Kṛṣṇa orders him, he must fight. He does not fight out of his own will, because a Vaisnava’s natural instinct is not to do harm to anyone. However, when a Vaisnava knows that Kṛṣṇa wants a particular thing done, he does not care for his own considerations. In any case, everyone has some particular type of duty, an occupation. If we perform our occupation in the worship of Kṛṣṇa, our life will be perfect. This is also the instruction given in SrimadBhagavatam (1.2.13):
atah pumbhir dvija-srestha varnasrama-vibhagasah, svanusthitasya dharmasya samsiddhir hari-tosanam
“O best of the twice-born, it is therefore concluded that the highest perfection one can achieve by discharging the duties prescribed for one’s own occupation according to caste divisions and orders of life is to please the Personality of Godhead.”
Source : A.C. Bhaktivedanta Swami Prabhupada (2007 edition), “Teachings of Lord Kapila, The Son of Devahuti”, Page 151