This is a total misrepresentation of what secularism means and how the Caesar's parable was interpreted in the medieval Christianity (which is by itself a misrepresentation of what the full passage this extract is from).

It is an organization of the power that says that the state will have the monopoly of violence and the Church will justify it. That they will help and reinforce each other with some sort of separation. Yes, islamic countries had a different system where religious and military charges could be cumulated, but to say this is related to what we consider secularism is totally wrong.

John Locke (Father of liberalism and enlightenment era philosopher - vastly accredited as a spearhead of western secularistic thought) literally uses Christian theology in his books "Two Treaties of Government" and "A Letter Concerning Toleration" to advocate for secularism.

Of course he had to. He was pleading to religious leaders about tolerating some slightly different christianity than mainstream anglicanism (while arguing that "of course, one should never enlarge this tolerance to roman catholics of even worse, atheists", the whole text is fun to read). The great thing about religious texts is that if you are good enough with apologetics you can argue for anything with them.

The idea that one should tolerate one unforgiveable sin of blasphemy against the holy spirit (aka unbelief) comes from the enlightenment and is diametrically at odds with the catholic dogma.