Their point is that despite your subscription to reason, without exposure to other cultural norms, you may be blind to what Christian values you live by. Becoming aware of them can help self evaluation of your ethical framework
For that matter, reading the Christian scriptures through a historical lens reveals a very different kind of thought than the modern version of Christianity and Judaism. It takes a huge amount of effort to read these documents in context; just reading them in the original languages is hard enough. But the past is a very foreign country and they see things very differently there.
No one is asking you to believe in anything, but it's self-limiting to refuse to engage with works of historical/cultural importance.
However I reject the idea that engaging with religious texts is insightful and something to promote