The Indian philosophical process evolved for a period of nearly thousand years in a dramatic fashion. The period between 600 B.C.E. and about 200 C.E. is called the Epic Era, when philosophical discussion and speculation took place at a steady pace.  The ideas of Upanishads were carried further and fine-tuned with the appearance of intensely philosophical literature called Darshanas. 

The predominant religion then was sacrificial Hinduism, but even then the tolerance and understanding for dissenting thought existed. How else could one explain the emergence of radically different thought processes on the part of Buddha and Mahavira, who denied the sanctity of the Vedas and defied existing practices of casteism? The Vedic sacrificial Hinduism, dominated by Brahmins and Kshatriyas, embroiled in caste differentiation and discrimination, did not persecute the unorthodox and nonconforming thought process of the budding new religions of Buddhism and Jainism. 

Buddha and Mahavira were not the only ones to propose different philosophies during their era. A rival branch called Ajivikas founded by Mahavira's rival Goshala Maskariputra was also popular at that time. Ajivikas did not believe in karma and thought that the destiny was predetermined and could not be changed. Goshala's followers were atheistic and believed that it was impossible to influence the course of transmigration and destiny in any way. There were also several other religious contemporaries to Buddha and Mahavira in the spiritually rich 6th century B.C.E. They were all rebelling against the rigid class-oriented sacrificial rituals of the Brahmanical Upanishads and the Vedas. Thus Purana Kassapa taught a no-action theory called akriyavada. He once explained to the Magadha king Ajatashatru that the soul is not affected by any deed (karma), be it good or bad. Sacrifices and self-mastery, even robbing and destruction have no effect on the passive soul. Only the infinite mind could comprehend a finite world. Another teacher of the same period was Pakuda Katyayana, who also taught that the soul was superior to good and evil, thus unchanged or untouched by it. He classified everything into seven categories, i.e. earth, water, fire, air, pleasure, pain and soul, which were eternal. Hence Mahavira and Buddha called his doctrine Eternalism. Both Kassapa and Katyayana believed that the soul existed independently of the body and thus unaffected by karma.

Ajita Kesakambalin, another contemporary of Buddha went further and taught complete materialism. He did not believe in the afterlife and considered death as the final phase of all souls, fools as well as the wise. Sanjaya Belatthiputta was the leader of the agnostics (ajnanavada). Upanishads over millenniums had proposed opposing theories of body, soul, karma and transmigration. Sanjaya rejected all the contradictory theories of body and soul. He thought it was better to reject them all instead of adopting one folly over another. His followers circumvented and avoided speculative questions rather than trying to answer them, hence were accused of being ignorant (ajnana). Sanjaya asked his followers to focus their attention on the attainment and preservation of mental equanimity.

The school of materialism called the Charvaka (also called Lokayat) did not believe in the cycle of rebirth and transmigration. The frugal virtues of Buddhism and Jainism were rejected and followers were encouraged to reject all religious observances and make the most of life's pleasures! The original writings of the Charvaka are lost to us, though Brhaspati is said to be its author. We know of the existence of such a system more from comments its opponents, who were Buddhist, Jain and Brahmanical scholars. The Charvakas totally disregarded Vedas as false and even referred to the authors of the three Vedas as demons. They were of the belief that the sacrificial rituals benefited only the priestly class (they might have touched a raw nerve here), and advised its followers that there was no afterlife one should be concerned of during this lifetime. It supported materialism and atheism. The sect existed in one form or another even during the Vedic Era (2500-600 B.C.E.) but later fell into disrepute. With the establishment of Buddhism and Jainism, the Charvakas were looked down as fringe organizations and were ridiculed and shunned but not persecuted. Madhava Acharya wrote an extensive review of the Charvaka system in the 14th century C.E. in the Sarvadarshanasamghraha. 

Buddha challenged the Vedic practice of rigid sacrificial rituals and the practice of caste systems. Following this Mahavira and Goshala also took extreme positions in their quest for answers and paved the way for ordinary people to dissociate themselves from the hierarchy of the powerful practitioners of Vedic rites and discrimination. The discussion became intensely philosophical and atheistic. The doctrines of the six Darshanas (namely Nyaya, Vaisheshika, Sankhya, Yoga, Mimamsa and Vedanta) were assimilated and as seen in the Sankhya system even the existence of a Creator was questioned. Creation was explained in more logical and metaphysical theories. Teachers like Kassapa and Katyayana questioned the role ofKarma on the soul. However, the very essence of Upanishads of realization of Brahman was never completely abandoned, at least by the major teachers. Buddha's Buddhists, Mahavira's Jains and Goshala's Ajivikas practiced ascetism and meditation in some form or another but did not discount God entirely. They admitted to a presence of supernatural beings though of limited powers. They also believed in the fundamental doctrine of transmigration and karma. 

While the Buddhists and Jains believed that karma could be altered by the deeds in this life, Ajivikas believed that karma was entirely predestined (niyati) and could not be altered at all. Until devotional Hinduism came to be practiced by the masses, many teachers taught skepticism and even materialism (materialist Ajita Kesakambalin and atheist Sanjay Belatthiputta). Skepticism about rebirths and status after death arose as seen in many questions posed to gods in the Upanishads in this subject matter (e.g. Katha Upanishad, with questions about death asked by Nachiketa to Yama). Even Mahavira and Buddha agreed that there could be no final answers to some of the difficult questions of cosmology, ontology, theology, and eschatology. 

The Epic Period was a golden era in Indian philosophical thought because of the tolerance of different opinions and teachings. In a wide scale, between sacrificial rituals to please the gods and complete agnostic and materialistic practices, existed many disparate and dissimilar ideas. The freedom to practice and follow any one of these teachers was unquestioned and unhindered. There was not only a freedom of religion but also freedom to practice a religion in any manner, without compulsion and decree or ordainment. This was the beginning of secularism and freedom of religion.

Most of the above teachers were contemporaries. They argued, discussed, rendered discourses and critiqued each other's theories. Above all they tolerated each other. Some of them, like Buddha and Mahavira, left their indelible marks on society and became leaders of new faiths. Others dwindled and became remote characters in the saga of history. But they all had their chances, un-hindered by bigotry or persecution just because they thought differently. 

Modern day Hindus, especially ones with extremist predilections and views, have a lot to learn from this era of Indian history. Civilized discussion and disagreement should be accepted but persecution should be condemned. If Christianity and Islam had been introduced at this juncture of India's intellectual development, along with Buddha, Mahavira and Goshala, they also would have been accepted and not considered rebellious. It is even possible that Christ and Muhammad would have been assimilated as Vishnu's avatars. Jesus would have been a shoo-in but even the militancy of Muhammad would not have mattered. Muhammad could have learned to get along with others. Mythology and Hindu Puranas have many instances of militant heroes, who were still considered to be leaders. After all Parashurama, who is credited with annihilating generations of Kshatriyas, is still considered as one of the avatars. An era that accepted atheistic Charvakas, eternalists and materialists would have had little problems with Christianity or Islam.

There is a Sanskrit shloka that summarizes the philosophy and the attitudes of Hindus regarding their spiritual journey and all other religions. It translates as follows: