“To some it is given by the Holy Ghost to know that Jesus Christ is the Son of God, and that he was crucified for the sins of the world. To others it is given to believe on their words, that they also might have eternal life if they continue faithful.” (D&C 46: 13-14)
If some have to "believe" something as fundamental as that Jesus is the Son of God, then it seems logical to assert that even more are going to have to believe that Joseph Smith was a prophet - and myriad other uniquely Mormon teachings.
I have no problem with saying, "I know” about a lot of things - but there also are things that I still believe without yet knowing. I would love to hear more people stand in front of the congregation and say, “I believe . . .”- or even, "I want to believe . . ."