And now because of their unbelief they could not understand the word of God; and their hearts were hardened. (Mosiah 26:3)
You can choose not to believe. One consequence of that choice can be an inability to understand the things of God. As Elder Packer taught, the skeptic will never know what we know because he will not meet the requirement of faith.
Belief is a choice.
Notice also the next verse: because they did not believe, they refused to be baptized, so they never learned the things you can only learn by entering that covenant and receiving the Holy Ghost. Just as belief leads to positive action, non-belief holds you back from taking those actions which would lead to greater belief.