"In matters of personal belief, how do we know what really is true? I testify that the way to know the truth about God is through the Holy Ghost. The Holy Ghost, the third member of the Godhead, is a personage of spirit. His work is to “testify of [God]” and to “teach [us] all things.”
However, we must be careful not to constrain His influence. When we do not do what is right or when our outlook is dominated by skepticism, cynicism, criticism, and irreverence toward others and their beliefs, the Spirit cannot be with us."
Elder Robert D. Hales, "Seeking to Know God, Our Heavenly Father, and His Son, Jesus Christ," October 2009