John Owen is again helpful to us in this matter.
When we realize a constant enemy of the soul abides within us, what diligence and watchfulness we should have! How woeful is the sloth and negligence then of so many who live blind and asleep to this reality of sin, There is an exceeding efficacy and power in the indwelling sin of believers, for it constantly inclines itself towards evil. We need to be awake, then, if our hearts would know the ways of God, Our enemy is not only upon us, as it was with Samson, but it is also in us.
Jerry Bridges, The Discipline of Grace, p. 208