(Srila Prabhupada translating, 1974)
"One should accept this opportunity to return home, back to Godhead, very enthusiastically. Without enthusiasm, one cannot be successful. Even in the material world one has to be very enthusiastic in his particular field of activity in order to become successful. A student, businessman, artist or anyone else who wants success in his line must be enthusiastic. Similarly, one has to be very enthusiastic in devotional service. Enthusiasm means action, but action for whom? The answer is that one should always act for Kṛṣṇa—kṛṣṇārthākhila-ceṣṭā (Bhakti-rasāmṛta-sindhu).
In all phases of life one has to perform devotional activities under the direction of the spiritual master in order to attain perfection in bhakti-yoga. It is not that one has to confine or narrow one's activities. Kṛṣṇa is all-pervading. Therefore nothing is independent of Kṛṣṇa, as Kṛṣṇa Himself states in Bhagavad-gītā (BG 9.4):
mayā tatam idaṁ sarvaṁ
na cāhaṁ teṣv avasthitaḥ
"By Me, in My unmanifested form, this entire universe is pervaded. All beings are in Me, but I am not in them." Under the direction of the bona fide spiritual master, one has to make everything favorable for Kṛṣṇa's service. For example, at present we are using a dictaphone. The materialist who invented this machine intended it for businessmen or writers of mundane subject matters. He certainly never thought of using the dictaphone in God's service, but we are using this dictaphone to write Kṛṣṇa conscious literature. Of course, the manufacture of the dictaphone is wholly within the energy of Kṛṣṇa. All the parts of the instrument, including the electronic functions, are made from different combinations and interactions of the five basic types of material energy—namely, bhūmi, jala, agni, vāyu and ākāśa. The inventor used his brain to make this complicated machine, and his brain, as well as the ingredients, were supplied by Kṛṣṇa. According to the statement of Kṛṣṇa, mat-sthāni sarva-bhūtāni: (BG 9.4) "Everything is depending on My energy." Thus the devotee can understand that since nothing is independent of Kṛṣṇa's energy, everything should be dovetailed in His service.
Endeavor executed with intelligence in Kṛṣṇa consciousness is called utsāha, or enthusiasm. The devotees find the correct means by which everything can be utilized in the service of the Lord (nirbandhaḥ kṛṣṇa-sambandhe yuktaṁ vairāgyam ucyate). The execution of devotional service is not a matter of idle meditation but practical action in the foreground of spiritual life.
(Nectar of Instruction 3, Purport)