While those committed to extreme forms of secularism argue religion, especially Christianity, must be abolished it’s ironic that at the same time they adopt many of the characteristics of the very belief systems they wish to destroy. Secularism becomes a new religion. 

As to the explanation, one reason offered by Adam Zamoyski in his book Holy Madness tracing the impact of the Enlightenment and radical forms of humanism is because abolishing religion, especially Christianity, results in a spiritual and moral vacuum yearning to be filled. 

Drawing on the example of the French Revolution (1789-1799) Zamoyski writes “Man seeks ecstasy and transcendence, and if he cannot find them in church, he will look for them elsewhere”.