Peter had great faith in the Lord, but it grew over time. His confidence in God and his resolve to stay true increased with experience and diligence. Some of Christ's disciples found that their faith, though "on," was insufficient to cast out a devil. Christ taught them that further action was needed if they were to increase their faith to the necessary level. Though their faith was strong enough to leave everything they had behind to follow their Lord, Christ's truest disciples asked Him how they could increase their faith.
Faith can start out smaller than tiny seed, and yet have great power even at that level. However, a seed is relatively fragile. To realize the greatest power of faith such as that seen in the great Apostles and prophets of the past and in our day, we must be true to the faith that we have. We must build our faith over time through diligent, unwavering obedience.
Those deceived by the light switch paradigm see greater value in "temporary" disobedience. It seems not as risky to simply turn the switch off for a short burst of carnal satisfaction, since the switch can be turned back on at any time. Though a child of God always has the right to choose to believe, simply making the choice to believe is only the first step, rather than the end goal.
Those who are awarded eternal life will be known by the Lord for the strength of their faith. Christ will present them before the Father at the last day as having proven their commitment to the truth, come what may. If the Lord cannot be sure that we will be true to Him, why would we feel worthy to live in His Kingdom? Even if we believe in Him, our belief and our confidence in that belief may not be strong enough to endure the most difficult trials of our faith. This strength of faith requires work, humility, patience, and grace to achieve.
The purpose of life, then, involves the training and testing our faith, refining us (if we choose to allow it) into one who has a perfect faith, strong enough to guide our actions even in the face of the greatest opposition that can be combined against us. This life is a school, a testing ground, a refiner's fire. Bad things have to happen to good people, temptations must come, trials must be endured. If there were none of these, how would our faith grow?