Bhakti, that is devotion, was born of the worship. Worship was nothing but an expression of theism which meant belief in the existence of God.
In Upanishad Brahma has been described as the God. However, in the post-Vedic period theism manifested itself in the worship of the prevalent folk deities like serpentine (naga), demon (yaksha), etc.
It was this worship that lay behind bhakti.
This bhakti attracted the attention of the common people. For, to them it was not possible to perform the rituals that became the most important part in the performance of religion.