The Later Vedic period saw a change in the religious life of the people. Simple prayers and simple sacrifice of the Rig-Vedic period were then replaced by elaborate rituals and scarifies.
In a word, sacrifice became much more important than prayers. The elaborate ceremonial in the matter of worship gave the Brahmanas a predominant position in the society.
For, they were considered to be the only persons capable of conducting the minute rules of ceremonial worship. However, in this period, some of the deities of the Rig-Vedic times lost their previous importance.
On the other hand, Prajapati, ‘the lord of creatures’ occupied supreme position. Besides, some minor gods of the Rig-Vedic days, such as Rudra and Vishnu gained prominence in the Later Vedic period.
In the Later Vedic period the performance of rituals was the most essential part of the Brahmanical religion. Side by side there was also reference to theism in the Later Vedic texts.
That is to say, the idea of belief in God came to be emphasized. Thus it has been stated in the Upanishad that Brahma was the Creator.
Brahma is responsible for the creation of the earth. The life on the earth is also his creation. In other words, it may be said that in the Upanishad Brahma has been described as the God. This is how the idea of God or theism is: born.