India celebrates itself as a nation of unity in diversity, but where does that sense of unity come from? One important source is a widely-accepted narrative called the "bhakti movement." Bhakti is the religion of the heart, of song, of common participation, of inner peace, of anguished protest. The idea known as the bhakti movement asserts that between 600 and 1600 CE, poet-saints sang bhakti from India's southernmost tip to its northern Himalayan...