First Nephi 2:16

And it came to pass that I, Nephi, being exceedingly young, nevertheless being large in stature, and also having great desires to know of the mysteries of God, wherefore, I did cry unto the Lord; and behold he did visit me, and did soften my heart that I did believe all the words which had been spoken by my father; wherefore, I did not rebel against him like unto my brothers.

A believing heart is a gift. As Alma taught, we begin with just a desire to believe. If we ask God in faith to help us believe, he is quick to answer such a prayer.