So far, we have seen two of the three enemies of God’s people–the world and the devil. This post will begin looking at the enemy within–sin. The devil only has the power to tempt and the world only has the power to conform because of our own sinful hearts. We can never say, ‘the devil made me do it’ or ‘I got caught up in following the world’ as if we are not responsible. On the contrary, we freely chose to do what our sinful hearts longed to do–sin. If we cannot understand the sinfulness of our own hearts, then we will never succeed in spiritual warfare.
Wayne Grudem helpfully defines sin this way: “Sin is any failure to conform to the moral law of God in act, attitude, or nature.”
The Bible presents many actions as sinful. That is to say, how we live can either be righteous or sinful before God. Often then, the Bible calls to live for God, apart from sin: “Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds. They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart. They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity. But that is not the way you learned Christ!—assuming that you have heard about him and were taught in him, as the truth is in Jesus, to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, and to be renewed in the spirit of your minds, and to put on the new self, created after the likeness of God in true righteousness and holiness” (Eph 4:17-24).
We often think of sin this way; it’s about what we do. But the Bible says that sin also comes from our attitudes, from the intentions of our hearts. So, Jesus says in Matthew 5:27-28, “You have heard that it was said, ‘You shall not commit adultery.’  But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
And more than just attitudes or motivation, sin is not just acting wrongly, but also failing to act in the right way. There are sins of commission as well as sins of omission. James says, “So whoever knows the right thing to do and fails to do it, for him it is sin” (4:17).
1. Sin’s Nature
Defining sin in relationship to God reveals its true nature: sin is essentially rebellion against God.
Sin is the willful and selfish rejection of God’s rule while choosing to live independently of him.
Romans 1 says, “For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things” (1:18-22).
Sin is always fundamentally and primarily against God
When we sin, people are affected. We are affected when we sin. Sin has consequences. But, in the end, our sin is ultimately against God.
Psalm 51:4 – Against you, you only, have I sinned and done what is evil in your sight, so that you may be justified in your words and blameless in your judgment.
2. Sin’s Pervasiveness
Humanity’s sinfulness extends to every part of his being; nothing in him has been unaffected by sin. We act, think, feel, desire in ways that are tainted by the sinfulness of our hearts. This has some far-reaching implications.
Sin’s corrupting effects have touched every part of our being
Romans 3:10-12 – as it is written: “None is righteous, no, not one; no one understands; no one seeks for God. All have turned aside; together they have become worthless; no one does good, not even one.”
Because of sin, apart from God, we are incapable of pleasing or obeying God.
Even seemingly “good” works are tainted by sin.
Romans 8:7-8 – For the mind that is set on the flesh is hostile to God, for it does not submit to God’s law; indeed, it cannot. Those who are in the flesh cannot please God.
Romans 7:23 – I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members.
Not only are we therefore enslaved to sin, but we are completely responsible before God for our sin.
Romans 14:12 – So then each of us will give an account of himself to God.