For me, it was hearing a talk by a creationist just after I'd graduated from university. He said something about how genetic information gets worse overtime with mutations. Somehow this made everything click. I feel that I'd known all of the pieces already and just needed someone to show me the whole picture. I knew that you can't have complex things arising by chance (and natural selection) from simpler things - that just doesn't work. I didn't know much about DNA at that point, but I did know that mutations were random things, not something intentional like editing a book to fix mistakes and make it better.

Mutations would mean that genetic diseases are increasing, and fit exactly with the idea that as one goes back in time the genome would be more and more perfect: just what Genesis says. God created Adam and Eve perfectly and because of their perfect genomes they lived a long time, even after sin and death entered the world. Everything fit and I could discard evolution. The Christian world view already explained life far better than any other one I'd encountered, and now it worked with our origins too (on a scientific basis). [Kind of coming back full circle since the Christian world view is one of the things that started science in the first place, although it's getting harder and harder to find people who say this due to the political correctness and revisionist history that our society embraces so much. What would be so bad about admitting that the Christian world view was a necessary prerequisite for science to be born?]

This "already knowing something, but not seeing it clearly" also happened to me about 5 years ago with pacifism. I knew all of the pieces, but I wasn't really sure about how wrong it would be to be in the military as a Christian, how wrong it would be to kill others to protect one's tribe/country/economic interests/freedom. I had some inklings and ideas, but it was a sermon that I heard about pacifism that put it all together with the nature and character of Jesus. Suddenly everything clicked and I saw the world in a new way -- but, on the other hand, I'd always known this, I already knew all the pieces, I just didn't see how they all worked together. It's a fascinating experience.