You are the guard. Living by faith is about guarding your heart. So, in practical terms, how do you 'guard your heart'? By beeing careful of three things: 1) the stuff you say; 2) the stuff you focus on and 3) the stuff you do. In other words, your heart is about the words you say (mouth), the things you see (eyes) and the things you do (feet). If you are walking by faith, then you are the guard.

Proverbs 4:23-27