Since the dock is so 'high' above the water, simply throwing a life preserver down to the people will not do. As long as the person hangs on to the preserver, the fact that he fell in, no longer matters. It is now in the past and forgiven. He had it, but didn't heed it. God is the One who justifies and saves.
Faith is the agency of that salvation.
You see, they have thought they had it all. But they are just as lost as the rest. They must get rid of their pride and SELF-righteousness. They have supposed that their 'law' indicated righteousness, not realizing it actually displayed their sin. In other words, he is introduced, for the first time, to the concept of faith.