A common myth is that secularism is what is left when we have stripped all former belief in superstition and falsity. The reality is that secularism is a system that was built up slowly over time. How did men construct secularism?
First, our idea of the relationship between the individual and the community changed. We used to believe ‘we are all in this together’, that God will bless or curse us as a community—think about the plague in Thebes in “Oedipus Rex” or the consequences of Achan’s sin in Joshua chapter 7. If this is true, then what you do is not just your business, but mine—your heresy might lead to my home being burned down as God gives our enemies victory over us, it might lead to the death of my children in a plague, etc. Because your heresy or sin could destroy the community, it must be stamped out. Today we see ourselves not as a community but rather as a collection of individuals and we don’t believe in collective consequences for individual choices. (Note: this belief is certainly in tension with the pandemic, but it is accepted opinion when it comes to issues of gender, how we spend our time and money, our private worship, etc.) If this is true then it follows that if what you do doesn’t directly hurt me I should not interfere with your choices. As a society we believe that my private opinions don’t hurt you nor do yours hurt me, therefore everyone should be able to believe what they wish to believe.
Second, we changed our idea of reality. We used to believe we inhabited a cosmos in which the visible (i.e. the immanent) was only a small part and it was regularly invaded and influenced by unseen, spiritual forces (i.e. the transcendent). Whereas today we believe the universe is self-contained, operating on the basis of fixed, unchanging, mechanical laws.
Finally, because our view of reality changed we needed to change our view of meaning to fit that reality. Humans used to look for meaning in transcendent things (i.e. things like Heaven or Truth, things that transcend the immanent/physical world.) Though people disagreed about the goal, everyone believed that humanity was made to be something greater than it is. Because we couldn’t agree and because we couldn’t seem to attain it, we lowered the bar and began to focus on physical, human flourishing. People have always sought this, but now many seek this exclusively (Taylor calls this exclusive humanism). Think of your secular neighbors—many seek their meaning exclusively in the things of this world, with no reference to higher or spiritual things. But meaning, by itself, is a mirage—in itself it has no lasting good. Men have died for their country, for their family, for God, but no one has died for “meaning” and no one can live for meaning. So secularism’s quest for meaning apart from any concrete, objective, transcendent good is bound to end in failure and frustration.
Secularism creates a world wherein people believe that only the physical/immanent exists, so they seek their meaning only in this and are free to do so because we all agree that they exist primarily as individuals and not as members of a community wherein their choices could have damaging consequences. This is a belief system with multiple components that developed slowly over time; it is not simply what is “left over” when we discard primitive beliefs.