According to Raymond Treat, the word "believe" is one of the most commonly used words in the Christian vocabulary. "Believe on the Lord Jesus Christ and you shall be saved" is frequently heard--"only believe." It seems that no other action is required according to the way some use the word "believe." However, Helaman 14:13 gives us a different definition: "And if ye believe on his name, ye will repent of all your sins, that thereby ye may have a remission of them through his merits."
According to this definition, if we do not repent of all our sins we cannot honestly say we believe. This verse adds a dimension to the word "believe" that goes far beyond the popular use of this word. [Raymond C. Treat, "Approaches to Studying the Book of Mormon," in Recent Book of Mormon Developments, Vol. 1, p. 156]