"Secular humanism says that science and rational thought will enable us to find every gem of truth that’s worth knowing."

I do not think this statement is true. Secular humanism merely describes a method for reaching an understanding about reality. It is an approach to life that tries to be positive, rational, realistic, and open-minded. It says nothing about the ability of the process to successfully achieve these understandings. It takes a naturalistic approach to understanding reality and excludes supernatural explanations (until such time as the existence of anything supernatural can be supported by any credible evidence). Hence its appreciation of science. I do not think any rational secular humanist would believe every truth worth knowing is within our ability to acquire or comprehend.

In my experience it is only some of the religious persuasion that claim they already know with absolute certainty all the truths worth knowing. It inevitably follows that a good dose of faith is a prerequisite to understanding these so-called truths.