According to the bible  (Hebrews  11.1) faith is being sure what we hope for and are certain of what we do not see. In other words , faith is knowing what God has said is true , and therefore trusting God with our whole heart, soul and mind so faith is not just something we have in our heads .if we truly know that what God has said is true, then we will believe it with our whole being .and we will behave accordingly!we can't fool God by saying that we believe something in our heads ,but not behave in a way that shows that we really and truly believe it . faith in God reveals itself  in how we live our lives.