The religious and philosophical tradition centered on the worship of Vishnu, who is considered the supreme Lord. It is related to the belief in avataras (divine incarnations of Vishnu). The most popular avatars of Vishnu are Rama, Krishna, Vasudeva, and the twin brothers Nara and Narayana. The most characteristic form of yoga in Vaishnavism is Bhakti Yoga (the yoga of devotion), which is theistic. The Bhagavad Gita is the oldest known scripture of Vaishnavism. It is associated with the great sage Ramanuja and the alvars (Vaishnava poet-saints). In modern times, Vaishnavism has spread around the globe, largely due to the growth of the International Society for Krishna Consciousness (the Hare Krishna movement), founded by A.C. Bhaktivedanta Swami Prabhupada in 1966.