Thus complex religion of Vaishnavism is often viewed as a synthesis of the worship of gods Vishnu, Narayana, Vasudeva and Krishna which is achieved by the time of Bhagavad Gita (c. 4th century BCE to 3rd century CE). Worship of Vishnu is called Vishnuism and the monotheistic worship of Vishnu was already well developed in the period of the Itihasas.