Lord Kṛṣṇa says in the Bhagavad-gītā that He is the source of everything, but envious and cunning people try to refute this fact. Thus He appeared as Lord Caitanya and taught that Lord Kṛṣṇa is the source of everything. There is no difference between the instructions of Lord Kṛṣṇa and those of Lord Caitanya. The object of worship is the same. Still, the unfortunate people of this age refuse to accept these teachings. Trying to give them Kṛṣṇa consciousness is, as the well-known expression goes, like "casting pearls before a herd of swine." The human beings afflicted by Kali-yuga are like a herd of swine. The Lord has shown them boundless mercy by widely teaching the science of devotional service to Kṛṣṇa, which is rarely attained even by Lord Brahmā. Yet because it has been so easy for them to come by this precious and rare commodity, they have abused the mercy shown them. This is another manifestation of their misfortune. By teaching them the science of self-realization, Lord Kṛṣṇa has twice personally tried to save the people of this age from groveling in carnal pleasures, and both times they have converted those divine instructions into a means and an excuse for pursuing sense gratification.
When presented with a colorful glass doll and a diamond, a child will naturally be attracted to the doll and not the priceless jewel. Similarly, the people of Kali-yuga, endowed as they are with limited intelligence, have rejected the priceless diamond of devotional service to Kṛṣṇa and instead chosen the cheap doll of fruitive activity and dry speculation. Just as the child cannot comprehend that the invaluable diamond can purchase many thousands of cheap glass dolls, so the less intelligent people of Kali-yuga cannot understand that kṛṣṇe bhakti kaile sarva-karma-kṛta haya: "By rendering transcendental devotional service to Lord Kṛṣṇa, one automatically performs all subsidiary activities."
Those who know the science of Kṛṣṇa consciousness automatically know of subsidiary subjects like fruitive activity, speculative knowledge, yoga, charity, penance, austerity, and chanting mantras. Lord Kṛṣṇa confirms this in the Śrīmad-Bhāgavatam:
By executing devotional service to Me, My devotees easily acquire everything that can be attained by performing penances, fruitive activity, philosophical speculation, renunciation, yoga, charity, religiosity, and other pious acts.