Exercise - meaning?!

[Fear not, only believe]

Hello everyone! What does the word exercise mean in this sentence?

"How can one come to possess great faith? Now listen, here is the
answer to that: First, the blade, then the ear, then the full corn in the ear. Faith must
grow by soil, moisture, and exercise."

[Raymott]

It seems like a pretty silly analogy, because plants don't exercise. Perhaps the author didn't know that, or accepted on faith that they do.
Exercising a faith means practicing it, putting it into use.

[Thavinct]

i think it goes further than the practical meaning of the word
probably symbolic