Question

how did secularism get translated into law ( answer in short )

Answer 1

With the 42 Amendment  of the constitution of India  enacted in 1976,the Preamble to the Constitution 

Answer 2

Secularism got embedded in the Constitution and became a law through articles,acts and amendments..

1.42nd Amendment Act of 1976 added the word SECULAR in The Preamble of Indian Constitution.

2.Article 15(4) said that the citizens are not to be discriminated by anyone on the basis of RELIGION,CASTE,RACE etc. This promoted Secularism.

3.Untouchability (Offence) Act of 1955 made untouchability as an Offence. This too promoted Secularism.