The Upanishadic texts (like some of the earlier Vedic texts) are primarily concerned with acquiring knowledge of the "soul", "spirit" and "god". The Upanishadic concept of God was abstract and philosophical. Different Upanishadic texts postulated the doctrine of an Universal Soul that embraced all physical beings. All life emanated from this universal soul and death simply caused individual manifestations of the soul to merge or mingle back with the universal soul.. The Hindu philosophy of the Atman and Paramatman finds its basis from this scientific concept.
As a corollary to this theory emerged the notion that even as individual beings might refer to this universal soul - i.e. God in varied ways - by using different names and different methods of worship - all living beings were nevertheless related to each other and to the universal God, and capable of merging with the universal god. Such an approach was not incompatible with secular society, and permitted different faiths and sub-faiths to coexist in relative peace and harmony.
In the course of defining their philosophy, the scholars of the Upanishad period raised several questions that challenged mechanical theism (as was also done in some hymns from the Rig Veda and Atharva Veda). If God existed as the unique creator of the world, they wondered who created this unique creator. The logical pursuit of such a line of questioning could either lead to an infinite series of creators, or to the rejection or abandonment of this line of questioning. The common theist solution to this philosophical dilemma was to simply reject logic and demand unquestioning faith on the part of the believer. A few theists attempted to use this contradiction to their own advantage by positing that god existed precisely because "He" was indescribable by mere mortals. But, by and large, this contradiction was taken very seriously by the philosophers of the Upanishadic period. The Upanishadic philosophers attempted to resolve this contradiction by defining God as an entity that extended infinitely in all dimensions covering both space and time. This was a philosophical advance in that it attempted to come to terms with at least the most obvious challenges to the notion of god as a human-like creator and did not require the complete rejection of logic.
In the very process of their questioning, (and albeit speculative reasoning about god), they had opened the door for rationalists and even outright atheists who took their tentative questioning about the role and the character of God as "creator" to conclusions that rejected theism entirely. But in either case, many rationalist and/or naturalist philosophical streams emerged from this initial foundation. Some were nominally theistic (but in the abstract Upanishadic vein), others were agnostic (as the early Jains), while the early Buddhists and the Lokayatas were atheists.
Buddhism, on the other hand is basically a religion of the mind, of the present moment awareness, of leading a virtuous and responsible life, and of the individual who is in search of an answer to the problems of change, impermanence and suffering within the confines of his own experience as a practitioner of the Eightfold path. The tenets of Buddhism are not centered around the concept of an universal supreme being, who in other religions, is responsible for