If we give people the liberties to freely sin (i.e legalization of gambling, prostitution), if not even encouraging that sin through legal provisions specifically advising that it is allowed, then they'll not only reject the Gospel because they'll have their soul sucked into addictions, mindless degeneracy, and a betrayal of God, but they'll then enforce that culture on their children and continue a cycle of disbelief among their families and communities because no systems will be in place to re-instate and remind them of the morality of God. God destroyed the two cities in the Book of Genesis, Sodom and Gomorrah, because their cultures promoted this depravity and despised His will. Secularism is a trait of all modern cultures that similarly promote and adhere to the same profligate sinfulness that God displayed His hatred of. God hates proud sin, and proud sin is the inherent end-point for secularism.
