Incredible: 1b - Hard to believe. New Shorter Oxford English Dictionary
It strikes me that Mormonism is an incredible religion, in the two-sided sense that it both hard to believe and hard not to believe!
I realize that to most non-Mormons this is untrue - in the sense that they find Mormonism one-sidedly hard to believe and easy to disbelieve - hard to accept and extremely easy to dismiss.
But, when not due to sheer ignorance, that attitude is often due to them being blinded by negative prejudice; because there are remarkable facts about Mormonism which it is hard to believe are not due to its being true.
There are at least four two-sidedly incredible aspects of Mormonism - that is they are both hard to believe, and hard not to believe.
1. Joseph Smith.
On the one hand, it is hard to believe that such an ordinary and flawed person as Joseph Smith should have been a prophet of God; on the other hand it is hard to believe that anybody except a prophet of God could have done what he did.
2. The book of Mormon
On the one hand, the convoluted story of how the book of Mormon came to be written is bizarre, unprecedented - in a word incredible; on the other hand, it is very hard to believe that a book of such length, quality, complexity could have been dictated verbatim and unrevised in extremely difficult conditions and in just a few months.
3. The organization of the LDS church
On the one hand, the piecemeal emergence of the Mormon church, the adoption of elements from various traditions, the revisions and corrections of doctrine and so on - all seem like ad hoc improvisations and strain credibility; on the other hand, the results were incredible: a church which commanded great strength and devoutness and expanded exponentially for 180 years and successfully scaled up from a few hundreds to many millions.
4. Mormon theology
Contradicts so much of the theology of the historical Christian church, and so profoundly, that it is very hard to believe that almost all Christians could have been so wrong about so many things for such a long time; on the other hand, the Mormon theology is so simple, systematic and also Biblically coherent that it is incredible that Christians could have failed to discover it for so many centuries.
I could go on - but I hope the point has been made that if Mormonism is given its due, then incredibilities abound, and are rather exactly poised!
It is incredible that something as incredible as Mormonism could be true: it is also incredible that something as coherent, as long-term successful, and as good as Mormonism could be false.