In the ancient days, people followed an independent religious philosophy related with Dravidian practices. Around 3rd Century B.C., Jainism and Buddhism came into the district. It was the Jainism that set foot in the region. It is believed that Ernakulam was a main Jain centre in the ancient days. Many Hindu temples in the region were originally Jain temples. The famous Kallil rock-cut temple near Perumbavoor was earlier a Jain shrine dedicated to Parswanatha, Mahaveera and Theerthankara. Jain settlements in the district have been completely disappeared except a few at Mattancheri region. Buddhism did not make much impact in the district unlike southern part of the state. Both Jainism and Buddhism declined consequent on the insurgence of Aryans and by the revival of Hinduism. Efforts of Srimad Sankaracharya and Bhakthi movement donated much for the promotion of Hinduism. Contributions of Thunchath Ramanujan Ezhuthachan, Melpathur Narayana Bhattathirippad and Poonthanam Namboodiri accelerated the Bhakthi movement initially in the central region of Kerala and later to the whole state.

Mahadeva Temple at Aluva, famous for Sivarathri celebrations and prithu-tharpana (a ritualistic offering to dead ancestors); Vamanamoorthy temple at Thrikkakkara, where Onam festival was originated; and Bhagavathy Temple at Chottanikkara where evil spirits are eradicated by exorcism are situated in the district. Many luminaries associated with the religious history were born in the district. Srimad Sankaracharya who laid foundations of the Indian cultural synthesis as early in 9th century, Shadkala Govinda Marar, a great musician who could sing Pallavis to six degrees of time, Perumthachan, the legendary master carpenter, who designed and built many temples of Kerala and carved many idols of deities were born at Kalady, Ramamangalam and Uliyannur in the district respectively.

Christianity and Islam came to Kerala through the Thrissur district. Malayattur Church near Kalady in the district is one of the earliest churches of Kerala established by St. Thomas, the disciple of Jesus Christ who preached the gospel of Christ and attracted many natives into Christianity in AD 52. The arrival of foreigners created a favourable atmosphere for Christianity. The Synod of Diamper that formally united the ancient Christian Church of Kerala with Roman Catholic Church and played a major role in changing the history of Kerala was held at Udayamperur of the district. The first Jewish synagogue of Kerala was also established in Kochi. The message of Islam too reached in the district during 7th Century A.D. Communal harmony and religious tolerance are the significant characteristics of the people of the district.
 
Among the population of the district, Hindu community divided into Brahmin, Kshathriya, Ambalavasis (Pushpakan, Chakyar, Poduval, Varier etc.) Nair, Ezhava, scheduled castes, scheduled tribes and a good number of other categories. Gowda Saraswatha Brahmins who have migrated from Karnataka and Goa also constitute a considerable share in the population of the district and they have their own temples. According to a survey conducted by the Census Department, the district has 1285 temples.