Sorry, it ain't in the Bible and as a principle it reverses the truth.
What? your favorite pastor quotes it often? Ask him for a chapter and verse before you believe him. It is not in the Bible.
The way you believe is the way you will live. Not the other way around. In fact, it makes faith a work, i.e it thinks that you can cook it up by practice. It does not always achieve its aim.
The above maxim is often quoted in justifying some practice of piety, in turning an adiaphora to be no more adiaphora. It is a ploy for slipping in a practice with the guise that it will protect your faith. Think of the Galatian heresy here.
I have seen such styles when I was an RC kid and it clouded the Gospel. Before the Gospel can affect you it has to first come to you propositionally declared.
Let me explain...