Why is it so important to gain my own testimony?

We all need a personal testimony of truth. As our faith develops, we necessarily rely on the words and faith of others, like our parents, teachers, or priesthood leaders (see D&C 46:14). But if we are solely dependent on one particular priesthood leader or teacher for our personal testimony of the truth instead of getting that testimony through the personal line, we will be forever vulnerable to disillusionment by the action of that person. When it comes to a mature knowledge or testimony of the truth, we should not be dependent on a mortal mediator between us and our Heavenly Father.

Elder Dallin H. Oaks of the Quorum of the Twelve Apostles