Above all others, ISKCON traces its religious influences to the 16th-century Bengali reformer Chaitanya (1486-1533), the Hindu saint who founded the Gaudiya lineage of Vaishnavism. Like others within the Hindu sub-tradition of Vaishnavism, Chaitanya looked to the God Vishnu as center of his theology. Three tenets mark the Chaitanya school of Vaishnavism -- or Gaudiya as it is often called, after the region where Chaitanya preached -- as unique, all of which also appear in the International Society for Krishna Consciousness: 1) the centrality of Krishna, 2) the identity of Chaitanya himself as an avatar or form of God, and 3) the place of sankirtan (public devotionalism). The wider Hindu tradition understands Krishna as one avatar (or divine form) of the god Vishnu, but for ISKCON devotees, Krishna is not only an avatar, but also the highest name and identity of God. As such, the Gaudiya sect centers its worship on Krishna, but understands itself as part of a broader Vishnu-centered tradition.
The centrality of Krishna is not only the hallmark of Gaudiya Vaishnavism, but the absolute core of ISKCON. Whereas nearly every Hindu, Vaishnava (Vishnu-worshipper) or otherwise, accepts Krishna as an avatar or appearance of the divine, followers of the Gaudiya sect look to Krishna as the "Supreme Personality of Godhead," that is the personality and identity of the one true God who creates, sustains, and will one day destroy the cosmos. Members of ISKCON, like others in the Gaudiya lineage, understand their founder Chaitanya as an avatar, a form of the divine, specifically as a manifestation of Krishna who appears to assist humanity. However, this recognition of Chaitanya's identity as an incarnation of God does not lessen the nature of Krishna as Supreme nature of God. Gaudiya Vaishnavism, and hence ISKCON, is strongly monotheistic.
Moving from the realm of theology to practice, ISKCON has adopted Chaitanya's teachings regarding what he called sankirtan, the public social expression of devotion. Like other 16th-century Hindu reformers, Chaitanya responded to both restrictive caste structures and regimented learning systems by emphasizing communal public worship wherein all devotees, regardless of caste or station, engaged in worship through the chanting of easily-learned mantras. Chief among these was the mahamantra, or great prayer, "Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare, Hare Rama, Hare Rama, Rama Rama, Hare, Hare," which would in the 20th-century give ISKCON its informal name, the Hare Krishnas. Chaitanya taught that devotional singing represented the best method of achieving transcendental consciousness for the present cosmic era, or yuga.
Chaitanya's approach to worship emphasized the direct ecstatic experience of God, unmediated by textual study or caste-restrictions. It appealed to a wide variety of Hindus, and even, the historical records indicate, Indian Muslims. Scholars have noted obvious parallels between Chaitanya Vaishnavism and Islam, particularly monotheism and the umma, the Islamic holy community comprised of all people, as well as the Sufi tradition of Islamic mysticism, which employs a method of chanting God's name in its worship, similar to that of Gaudiya Vaishnavism. These commonalities are all the more notable because Chaitanya lived during an era of Muslim political dominance of Bengal.
In the 1960s and 1970s, ISKCON fused Chaitanya Vaishnavism with the countercultural spirit of the West, creating an alternative religious culture. When A.C. Bhaktivedanta Swami arrived in New York City in 1965, he had no access to the elite social networks of New York and thus he preached in public parks, attracting the attention of the many youth. A year later when he relocated his movement to the Haight-Ashbury section of San Francisco, scores of young people flocked to the newly established ISKCON temple in rejection of what they perceived as the hegemony of materialistic American culture.
Bhaktivedanta's "Hare Krishna movement" -- as it was dubbed even before the official founding of ISKCON -- appealed to these countercultural youth because it simultaneously represented the exotic and the authentic. With his saffron robe and Indian accent, the elderly Prabhupada represented the ideal spiritual guru, and he offered a religious teaching that involved Sanskrit mantras and sacred texts, the cooking of Bengali cuisine, and the use of Indian music in worship. For the hippies who had rejected mainstream American culture, ISKCON offered a new religious path rooted in ancient Indian spirituality rather than the West.