Watch for Indwelling Sin (The Flesh).
We are born with a traitor within the walls of our heart. Indwelling sin is present at birth, and it will stay with us throughout our lives. Sin will betray us to our temptations. Sin will open the gate to any corruption from outside.
We don’t need Satan to sin. We can sin on our own. Look at Genesis 6:5: “The Lord saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually.” (Genesis 6:5, ESV) And what does God say after the Flood? “And when the Lord smelled the pleasing aroma, the Lord said in his heart, “I will never again curse the ground because of man, for the intention of man’s heart is evil from his youth.” (Genesis 8:21, ESV).
So then where do our sins come? What is the source of our anger or lust?
But each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death. (James 1:14-15, ESV)
In Southern California, powerful dry winds blow from off the of the desert and rage across the hills and through the valleys. After a few days of blowing, the summer brush becomes perfectly dry. To make matters even more dangerous, certain plants actually contain chemicals which ready them to burst into flames. Thus, by October of a dry year the least spark will explode in the grass. In moments, the hillsides will be raging with fire.
The sin of our heart is ready to burst into flames of lust and anger. In California, when the land becomes dry and dangerous, campfires are banned. Likewise, our dangerous heart is best kept from sparks if we want to avoid a fire.
As Thomas Manton writes: “We proceed every step to heaven by conflict and contest, because sin is always at hand, ready to assault us and taint us ; so that a serious Christian cannot but take himself to be still in danger.”