A.H. Strong uses the analogy of the coupling. The coupling joins a train of cars to a locomotive. The coupling has no power in itself. It cannot move a single car an inch. All the power is in the locomotive. But the coupling is the link by which the power of the locomotive is transmitted to the cars. Faith has no power in itself; it is not a ground of salvation; it is not a good work. It is merely that by which all the goodness and grace and glory of Christ comes to the sinner.