The situation seems to change a lot if we look at the cultural landscape of, e.g., 13th c. India. There, most if not all philosophers seem to have been genuine believers. They adored a God and wrote about their religious experiences. Why this change?
(I know, "religious feelings" is an ambiguous definition. But please bear with me; after all we all notice the difference between Rāmānuja and Vātsyāyana.)
- 1. Because the etiquette in the previous period was to keep one's religious experiences private. After all, even in the subsequent period, authors often tended to distinguish their religious works and their rational-argumentative ones.
- 2. Because the philosophical milieu of India before 800 AD (as a conventional date) was much more secular. Religion was wide-spread on a popular niveu (as can be seen by devotional texts such as the Purāṇas, and artistic manifestations), but much less so among philosophers.
- 3. Because later philosophers had to admit religion in their texts due to the influence of the increased political or social (or economical) significance of religious groups.
- 4. Because the "religious feelings" of earlier authors were less "personalised". Earlier authors worshipped impersonal entities, like the pursuit of epistemic validity, or dharma (either in the Vedic or in the Buddhist or Jaina sense). The difference is only in the fact that these bear no personal names etc.
- 5. Because the opposition between these two periods is just ill-funded. I am biased by my education, etc. Just like one can be "religious" without worhipping a god (as in most Buddhist groups), it does not make sense to speak of a religious or not-religious attitude. But, if so, how to rephrase the contrast?
Any other proposal to interpret this phenomenon?
On different concepts of deities, see also this post.