Faith works in a similar manner. We may feel that we could not believe any more strongly than we do. But as we live our faith over time, our faith grows even more. It isn't that we didn't believe with all our hearts and minds before, it is that our ability to believe has grown.
Peter seems a good example of this principle. From the beginning, he was willing to leave his life and livelihood, to sacrifice everything simply to follow his Savior. When others questioned the Savior's divinity, he unequivocally stated "Thou art the Christ, the Son of the Living God." Such strength of faith is a rare thing. Yet, in spite of this strength, he wavered when he tried to walk on the water to Jesus. He denied association with Christ three times during the Savior's final trials. After our Lord's resurrection, he returned to fishing for fish rather than men. Though Peter may have felt he had complete faith, his faith still seemed to grow with time. Shortly after Christ's ascension, he rejoiced that we was beaten and imprisoned for preaching of Christ. He immediately acted on a controversial revelation he received on the preaching of the Gospel to the Gentiles.
It seems clear that Peter's faith wasn't weak in his own eyes when he left his nets to follow Christ. Yet it seems clear his faith grew with time. Peter's capacity for faith seemed to grow with experience. His ability to believe ever more strongly--to be ever more convinced of the truthfulness of the Gospel--seemed to grow with time.