Orthodox Hindu Schools
As anyone familiar with the orthodox Hindu darshanas knows, a belief in God is not a central feature of all orthodox schools of thought. Two of the orthodox Darshanas in particular seem distinctly rooted in materialism: Samkhya and Vaisheshika. Those are discussed below.
Chattopadhyaya goes so far as to claim that:
“If the Sankhya philosophy were in the earlier times an explicit philosophical re-statement of the fundamental theoretical position implicit in Tantrism, and, if further, as we have aready tried to argue, the term Lokayata originally stood for the beliefs and practices broadly referred to as Tantrism, then original Sankhya may be viewed as the most important developmet of the Lokayata tradition in Indian philosophy. Silamka, the Jaina commentator, was justified in denying any basic difference between Sankhya and Lokayata. Sankara, too, made the Sankhya philosophers quote the authority of the Lokayatikas”