My impression may be mistaken, but it seems to me that Dr. Mattias Desmet may have overstated (perhaps due to keeping things simple enough to keep his book under, say, a gizillion pages) the supposed lack of rational foundation for religion.
In particular I am thinking of the articiulation of the rational foundation for the teachings of Jesus. And prior to Christianity and Jesus, there was a rational foundation for the builders of edifices like the pyramids. Both systems of thought, not considered as such at the time of their origin and flourishing, produced things of lasting human value that rermain with us to this day.
Connecting human beings has been the human condition, it would seem evident, across human history. Striving for meaning is intrinsic, I think, to the human condition. This appear to have been unchanging.
Perhaps Dr Desmet underestimates the quality of thinking and paints with a far too broad brush.
Truth-telling is a worthy subject to pursue, to be sure. And, frankly, the dangers of truth-telling predated the Enlightenment, certainly.
On the other hand, evil has been expressed by human beings in thought and conduct in far too deeply and broadly creative ways. The rise of science did bring a bias toward that which can be measured. That has always left much behind and pushed out of public discourse.
Desmet is saying that there are limitations to explain the natural world based solely on rationalist and materialist thinking. Because if you resort to rationalism, the output is people like Sam Harris, Richard Dawkins, Christopher Hitchens and Daniel Dennett. These debates took place back around 2010 when they emerged as the 'Four Horse Men'. Haidt also took aim at this group in his book - this debate is not new.