I have seen many testmonies that have not touched my heart, let's take an example of a Catholic priest who converted to islam, while to a certain extend I can accept what seems to be his genuine reasoning, while it looks genuine from the outset, he did not give a theologic justification for doing so, which makes me believe that he either did not truly understood his faith or he may not be who he claims to be. So why should I believe that testmony? We see thousands of Christian denominations each interpreting diferent Bible passages diferently, why should I believe one over the other? The same applies to individuals who convert, in unique cases like this we truly don't know the theologic reasons which lead someone to give up the fullness of Truth and follow a faith such islam, maybe GOD just wants the rest of us to back off and stop giving a hard time to all decent muslims... who knows, only GOD does. I guess the other option would be that GOD just needs someone calm and open to dialogue to infiltrate their ranks and helped them open their eyes to reasoning, after all we all children of GOD and GOD works in misterious ways.
I got to also say, I have watched much Muslim stuff on youtube that at first it shook my faith, but in the end such things only forced me to study certain matters further about my faith that i truly did not understood or was able to explain properly, hence why it nearly shook my faith.
I do think that protestants or people of weak faith would have struggled with such matters and possibly even abandoned their faith if they did not bothered to investigate why we Christians believe in such matters, I even go further to say the key to answer such matters for me was the apostolic church.
To give you some examples
The Holy Trinity
Deus Caritas Est
Errors in the Bible (Well I always believe in 2 thessalonians 2:15)
After truly investigating each of these and many others such extreme case, my Catholic faith has only been reforced more and more.