But someone will say,
"You have faith; I have deeds."
Show me your faith without deeds,
and I will show you my faith by what I do.
Man cannot look into the heart and determine whether or not there is really faith there. The Bible even talks about the possibility of deceiving ourselves about our salvation. Sin is such a blinding agent.
In this passage we are taught how we might "see" faith. It is the same way that one might say that they know that there is seed growing in the ground. If you look at a spot of soil, and there is nothing there, one might doubt whether or not there is really a seed that has taken root there. Especially if it is a seed that has been in the ground for a long time. But when a sprout shoots up and leaves begin to grow and the stalk of the small plant reaches toward the sun, then you know that there is a seed there that has taken root. The invisible has become visible.
Faith is the same way. It produces works, and we may show our faith both to ourselves and to others by the things that we do.