The Gupta period had witnessed the synthesis of Brahmanical Hinduism with heterodox creeds. The integration of various heterodox creeds like Saivism, Vaishnavism and Shakti cult with Brahmanical Hinduism, had marked the culmination of the Gupta period. The synthesis of heterodox creeds gave rise to neo-Hinduism or Puranic Hinduism, the flavour of which is still found in contemporary Hinduism. The ideal of Neo-Hinduism had almost changed the concept of Vedic Brahmanism, but the form however remained unchanged. Neo-Hinduism had shed its concept of multi-cult creators. The concept of three gods connected with life, death and destruction united together as "Trinity" or "Trayi" had first materialised during the Gupta Period. According to neo-Hinduism, the three gods Brahma-Vishnu-Maheswar were united in the trinity concept or Trayi. According to scholars, due to religious admixture of heterodox creeds, the concept of "monism" or the doctrine of different schools of thought had evolved during the Gupta period. Gradually Brahma, considered the God of creation, passed into oblivion. Only Siva and Vishnu dominated the neo-Hindu doctrine of the Gupta period. The Puranas were rewritten in order to accommodate Siva and Vishnu as the chief Gods. Not only they were considered the chief Gods, but were also attributed with extraordinary powers. Most of the Vedic Gods passed into oblivion and were replaced by new Gods according to the concept of neo-Hinduism. Gods like Siva, Vishnu, Kartikeya, and Ganesha who belonged to the heterodox creeds formerly replaced all the Vedic Gods. Thus due to religious movements during the Gupta Period, Hinduism became the vast mosaic of various religious patterns, combining religious ideas of both the old and the new.
One of the interesting features of religious development during the Gupta Period was the wide prevalence of worship of 'Shakti' or mother goddess. 'Tantricism', or the cult of Tantra that preached the worship of female deities, had initiated the fertility cult. Hinduism, prevalent in the contemporary Gupta Period could not escape the influence of the Shakti cult. Henceforth it gave rise to worship of several female gods, who themselves were considered the wives of the chief gods. The cult of mother Goddess became very popular. Originally "Shakti" was worshipped as the goddess of force in the form of Kali, Chamunda and Bhima. In the "Markandeya Purana" Chandi is described as a destroyer of Mahishasura, the symbol of evil. Gradually the character and concept mellowed down into goddess Shakti, who was considered the wife of Siva and mother of Kartikeya, Ganesha etc. The concept of Siva and Durga was very popular. Durga was the new form of Shakti. Two opposite cults were ascribed to the concept of Shiva-Shakti. Their violent aspect came to be known as Rudra or Ghora or Chamunda respectively. In their graceful manifestation they came to be known as Aghora Mahadeva and Uma. Uma, Haimavati, Durga, Kali were worshipped as the various manifestations of the wife of Shiva. Lakshmi was worshipped as the wife of Vishnu. Puranas were re-written to accommodate the new Gods and Goddesses in Hindu temples. The Puranas described the cult of neo-Hinduism and narrated the mutual relationship of various gods and goddesses, worshipped according to the concept of neo-Hinduism.
Prevalence of idol worship was another feature of Puranic Hinduism of the Gupta period. Specification of images of different gods and goddesses were incorporated from Puranas. The cult of Kartikeya and Ganesha was also very prominent during that period. The Kartikeya cult was popular among the Kushanas is evident from the figure of Kartikeya on the coins of Hubiskha, a Kushana chief. Kartikeya was originally considered the war God. Later he was included in the family of Shiva-Parvati. Ganesha was also unknown before the 300 A.D. In the Gupta period he became a popular God. Many images of Ganesha made of stone and terracotta, belonging to the Gupta period has been found. The concept of Goddess Lakshmi underwent an evolutionary change during the Gupta Period. Lakshmi was originally Gaja-Lakshmi and a solitary goddess. Later, according to the concept of neo-Hinduism, goddess Lakshmi was considered the wife of Lord Vishnu and the Puranas delineated the story of her birth from the ocean. Various virtues were added to her character and she was popularised as the goddess of wealth.
In the Gupta era, the Vedic form of worship by performance of yajna did not survive much. In order to make a synthesis with Vedic Hinduism, 'yajna' or sacrifice was retained along with idol worship. Yajna lost its prominence in the form of image worship. Bhakti or the devotion of the worshipper became more important. Still priests were needed to perform the worship, but the concept of priesthood lost its dominance due to the emergence of Bhakti cult. Worship of god henceforth became personal matter of the worshipper. Priests became irrelevant due to the decline of yajna or sacrifice. Therefore Almighty became a much more concern for the individual. Men started believing that he had four fold objects in life- Dharma, Artha, Kama and Moksha.
Apart from Hinduism, Buddhism also underwent transformation during the Gupta Period. Since Gupta rulers were tolerant towards other religious creeds, all religions flourished during that period. Nalanda received patronage from the Guptas. However the typical change that entered the folds of Buddhism, was the rise of Vajrayana or Tantric Buddhism. Buddhism adopted worship of mother goddesses like Tara. Buddhism also accepted the theory of incarnation of Buddha and thus prepared the way for assimilation by Hinduism. Jainism however remained much original in its creed. It received the patronage of the merchant community of western India. Jainism continued to flourish in south and western India, while neo-Hinduism became a dominating creed of north India.
The religious movement in India during the Guptas therefore is a synthesis and integration of different heterodox creeds with Brahmanical Hinduism, which ultimately led to a complete transformation of Brahmanical Hinduism prevalent in ancient India.