In the ancient days, the 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. Mathilakom in Kodungallur Taluk was a main Jain centre in the ancient days. It is believed that many Hindu temples in the region were originally Jain temples. Koodal Manikyam Temple at Irinjalakuda, Cave Temples at Irunilakkode near Wadakkanchery and Thrikkoor near Ollur were Jain temples. Buddhism did not make much impact in the district like southern part of the state. Both Jainism and Buddhism declined consequent on the insurgence of Aryans and by the revival of Hinduism. Efforts of Sri 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. Kunchan Nambiar, who born in the district, also contributed to the movement. Eminent personalities like Kodungallur Kunhikuttan Thampuram, Vemoni, Seevolli and Nattuvam Namboodiries also enriched the movement through their literary contributions. Sri Krishna Temple at Guruvayur was a centre of Bhakthi movement not only in the district but the whole Kerala also. Guruvayur has been declared as one of the national pilgrim centre of India. Similarly, Vadakkunatha Temple at Thrissur town is one of the national monuments.

Christianity and Islam came to Kerala through the Thrissur district. St. Thomas, one of the disciples of Jesus Christ came to Kerala by sea and landed at Kodungallur of Thrissur district (which was the capital of Cheraman Perumal, the ruler of Kerala and a major port in western coast) in the year AD 52. He preached the Gospel of Christ and attracted many natives into Christianity. Similarly, Cnana community reached Kodungallur by 345 AD from Syria.  They were allotted land by the Cheraman Perumal, the then ruler of Kerala to settle down and conferred on Cnanai Thomman privileges. The arrival of foreigners created a favourable atmosphere for Christianity. The message of Islam too reached Kerala in Kodungallur during 7th Century A.D. The first mosque in India was also built at Kodungallur in 629 A.D. This mosque is known as Cheraman Maliq Masjid.

According to 2011 census, the population of the district is 3110327.  Among them, the 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. To the Hindus, worship of god is imperative and hence temple formed an integral part of their daily life. According to a survey conducted by the Census Department, the district has 1513 temples.