The bhakti tradition enjoys a near monopoly over ways to be Hindu worldwide

Prothero tells us that "Today, the bhakti tradition enjoys a near monopoly over ways to be Hindu worldwide" (p. 58). Please identify major themes or characteristics of bhakti Hinduism? What is the bhakti form of Hinduism? What makes bhakti bhakti? Why did it become such a popular form of Hinduism - that is, why has it been so appealing to people?