There was no idol worship during Vedic period. Agnihotra or Yajna were performed. In Yajna, God is worshiped by honouring fire and fire is the vehicle that takes the request to God. However, Vedas contain hymns (mantras) to worship deities who number around 33 koti including Indra, Agni, Varuna, surya, vaayu, Vishnu and Rudra (later became Shiva). Idol worship is the extension of Yagna which is allowed in Vedas. Earlier Hindus used to worship cave temples so there was no need to construct temples. The earliest example of this kind was a cave temple that was built in around 400 B.C. at Bhaje containing images of learned Parasurama with Indra and Surya.
Later, worship of Kubera, Rama, Kesava, Krsna, Vishnu, Mata and Shiva started. This led to image worshipping and building of temples. Hindu Temples are spread over entire Indian sub-continent. Many temples date back to thousands of years. Sri Adi Shankaracharya (8th century) - He went on to connect holy spots of India such as the 12 jyotirlingas, 18 shakti-peethas and four Vishnu-dhams to create pilgrim routes that defined India as a single land.
A large number of ancient temples are found in southern and Far East countries such as Cambodia, Thailand, Malaysia, Indonesia, Nepal, Sri Lanka, Bangladesh and Pakistan. It is interesting to know that Hindu temples are spread over now in over 43 countries.
Indian Express dated November 26, 2019 reported, A total 651 Hindu temples in India are designated as Centrally Protected Monuments under the Archaeological Survey of India (ASI). The largest number of Hindu temples under the ASI’s protection is in Karnataka, followed by Tamil Nadu, Madhya Pradesh, and Andhra Pradesh (Karnataka – 149, Tamil Nadu – 111, Madhya Pradesh – 96, Andhra Pradesh – 91, Maharashtra – 41, Kerala – 39, Rajasthan – 30, Uttar Pradesh – 24, Gujarat – 16, Odisha - 15, Bihar - 8, J & K – 8, Himachal Pradesh – 6, Chhattisgarh – 5, Uttarakhand - 5, Haryana – 2, West Bengal – 2, Delhi – 1, Goa – 1 and Jharkhand -1. (Source: Lok Sabha question)
Archaeological findings of temples date back to thousands of years. A few evidences of old temples are mentioned hereunder to highlight the grand heritage of oldest religion:
Popular Hindu Temples constructed before the birth of Buddhism & Christianity:
Popular Hindu Temples constructed before the birth of Islam:
Popular Temples constructed prior to Islamic invasion in India:
the Mahabharata .