Without a doubt, the thing in my life that has challenged every belief I have is the last 4 years battling with infertility. Perhaps because I am still in the midst of the fight, I still have days where it overwhelms me and causes me to lose faith in everything.
About 1.5 years into my battle, I had all but forgotten faith of any kind. I just couldn't be bothered with any sort of belief that supported my inability to have a family. I told a friend as much, and she responded by pointing out that faith is easy when things are going well, but when things aren't going well is when faith is truly tested. I took her words to heart and began reading the Tao Te Ching again. Only 5 verses in, I read something that would completely change how I looked at my infertility.
Up until that point, I assumed that this infertility was a punishment of some kind. I agonized over what act I had committed that was so horrible as to warrant denying me a family that I yearned for so deeply. This was amplified by watching (what seemed like) everyone and their dog accidentally getting pregnant all around me. I cried as the next person I deemed "unworthy" announced they were pregnant, and I remained barren as ever.
The 5th verse of the Tao Te Ching explains that nature (or the universe if you prefer) is neutral. It is not a judge, rewarding and punishing the behavior of each of us based on our actions. I found great comfort in realizing that these new mothers were not deemed more worthy than me, they simply randomly drew a different set of circumstances than I had. Once I got past this, dealing with the infertility became much easier. Honestly, I still have bad days, those times of self-pity, but I no longer play over my life wondering why I'm being punished, and I don't dwell on the "worthiness" of those who got their family easily.