- The Christian faith is true
- Doctrine X is part of the Christian faith
- Therefore Doctrine X must be true
In the meantime, we need to live on faith. That is we need to live as though Doctrine X is true even though we don't understand it yet. Sin darkens the intellect so knowing how to avoid sin in the area we are still trying to understand is a big help. We can think clearly and experience the benefits the knowledge without really knowing what we are doing.
What often happens is that we eventually find some very important and very beautiful reasons why Doctrine X is true. This ends up strengthening our faith. At a minimum we see wisdom in the faith. That Christian tradition knows more about what it means to be human than the modern culture does. But often we see more than that. We can see Love. We can see a Father God and a Mother Church raising us up in the faith.
For example, when I was a teenager there was the issue of premarital sex. The culture told us it was OK and the church told us it was not. With my teenage mind I really didn't see a big issue. I was firmly protestant at the time so contraception was OK. Really it was purely a matter of "Thou shalt not." So I didn't.
After a few years I did see why that was a good choice. I saw others commit this sin and bad things happened. It actually made their dating life less joyful. Their relationships remained shallow, It led them into bad marriages. It, also, destroyed their relationship with God. This took years but but eventually I became convinced based on evidence that this was just a bad life choice. Those who chose to follow the culture never arrived at that conclusion. They were just to deep in to have any perspective.
I can tell you that grew my faith. Everyone was saying that the church was crazy on this issue. They are still saying it. But I know better and I only know it through the grace of God. So I trust that crazy church because it saved me.
BTW, this is not something that changed at all when I became Catholic. Catholics and protestants differ on how we evaluate point #2. That is how do you know what is part of the Christian faith. But once you become sure of that then this all works exactly the same.