Let’s broadly consider scientism to be something like the belief that science is the best means toward truth or that Christianity represents an unimportant addition to human knowledge over and above secular science. Let’s further define science narrowly as the best-practice implementation in modern times, including peer review and so on. In this case, there are clear comparative problems with scientism, and we may clearly defeat the notion that Christianity adds only marginally to social order.
One key problem is that we know that scientists are motivated to publish or perish, and that false positives, over-fit, and replication problems abound. This is a world of might makes right and truth by the majority, not a moral or truthmaking place by any sensible analysis. Even among human systems, it’s plausible to find many better truthmaking systems. For example, one can argue that market incentives create better honesty that those incentives in the academic system of peer review.
Now, a scientist may claim that this is only a contemporary problem in the peer review process, but this is hardly a small issue. First, we would need to show that science can autonomously solve this problem which it has created. Secondly, it appears that contemporary science is the best in history, so we should doubt any other concrete implementation of scientism would be better. Third, we can step outside the scientific norm and look at the scientific best and see that it also fails.
Many secularists and scientists today would agree that the best arguments for secularism have been put forth by the Four Horsemen, including Dawkins, Harris, Hitchens, and the fourth guy. Specifically, many would claim that The Moral Landscape by Harris is the best there is to offer, with macroevolution as a theory of everything coming in second, well-represented by Dawkins. Yet we know that The Moral Landscape fails because it commits an is-ought fallacy and lacks a grounding for the identification of flourishing as good outside of evolution, which is itself without moral grounding.