"Faith is simply prayer."
- Martin Luther, A Simple Way to Pray
What can this mean?
If prayer is talking with God about what he and I are doing together, then actual praying is the concrete expression of faith in a God who is there, who hears me, and who responds. Just as the cliff diver expresses his faith that the water will be deep enough by diving, so does the Jesus-follower express their faith that God responds to prayer by praying. Conversely, the person who does not actually pray but says they "believe in prayer" is a person of little or no faith. Perhaps even unbelief.
In the act of praying I am a person of faith. To see a praying person is to see a person of non-theoretical faith. Faith, for them, is not some theory. It's also not faith in some theory. Prayer is a sign of faith in action, of an active faith. Only real believers pray. To pray is to trust God.
Are you a person of faith in God? Then you will pray, just as surely as rain causes the ground to get wet.
I have faith, therefore I pray.
Credo, ergo oratio.