The word ‘Secular’ was inserted into the Constitution of India by:

The word 'Secular' was inserted into the Preamble of the Constitution of India by the 42nd Constitutional Amendment Act of 1976 [1]. This amendment, enacted during the National Emergency under the Indira Gandhi government, also added the words 'Socialist' and 'Integrity' [1]. Before this amendment, the Preamble described India as a 'Sovereign Democratic Republic'; the 42nd Amendment revised this to 'Sovereign Socialist Secular Democratic Republic' [1]. While the Supreme Court had noted in 1974 that India was already a secular state in practice through Articles 25–28, the amendment made this status explicit in the founding document [1]. The 42nd Amendment is often referred to as the 'Mini-Constitution' due to the extensive changes it introduced. Although the subsequent 44th Amendment Act of 1978 reversed many changes made during the Emergency, it left the Preamble's new terminology untouched [1].

Sources

1. [1] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 5: Preamble of the Constitution > II Secular > p. 43