What does it mean to have a sin nature? According to the Westminster Shorter Catechism, 18, there are three particulars under the general head of Sin. 1. The guilt of the first sin. 2. The corruption of nature resulting from the first sin. 3. Actual transgression, or sins of act, which result from corruption of nature. [Shedd, Dogmatic Theology, Volume II, 169] I know of very few theologians or churches that would deny 1 or 3. We all admit that sin has entered the world. Moreover, who among us would deny that we actually transgress God's law more often than we would care to admit? It isn't point 1 or 3 that bristles the human ego. Rather, it is in 2 that the scandal resides. That all human beings enter the world with a corrupt nature as a result of the first sin seems completely irrational, unjust, and utterly absurd.
Jesus describes the human heart as exceedingly wicked, For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. Mt 15:19. This is the gospel according to Jesus, remember Him. He is the one who loves everybody unconditionally! Right? Wrong!
Men are born suppressing the truth of God in unrighteousness. (Rom. 1:18) Men do not honor God as God, but have become futile in their speculations. (Rom. 1:21) Men have perverted the image of God. (Rom. 1:23) All men are and behave unrighteously and none of them seek after God. (Rom. 3:10) All humans have lost their fear of God. (Rom. 3:18) Men are not willing OR able to subject themselves to God or do anything that pleases God. (Rom. 8:7) Men consider something as beautiful as the gospel to be utter nonsense and scandalous. (1 Cor. 1:18) Men are blind and ignorant to the gospel of Jesus Christ. (2 Cor. 4:4) Men are sworn enemies of God. (Rom. 8:6) Men are ignorant in their understanding of the gospel. (Eph. 4:18)
To deny the sin nature is to significantly diminish grace. It is astounding that God would do for us what He has given that we did to Him what we have. Man sinned against a perfectly holy God. Since God is perfectly holy, such an act of treason cannot possibly be fully apprehended. Sin was far more than just a mistake. There isn't an act of law-breaking that provides a fair comparison. God, being perfectly holy, cursed man for his act of ungodly rebellion. This curse was profound. It touched every part of the human person. Man was now infected with sin. Every part of his being was touched by this act. In addition, the curse came from God and only God could lift it. It has never been up to man to remove the curse himself, by an act of his own will or by doing enough good works. Read the paragraph above and check each reference carefully is that is how you think. Man is unders the curse of God because of the fall. Every part of his being has been made the enemy of God top to bottom. His will, his intellect, and his emotion are all infected with sin. That is just how sin is and that is exactly how the Scripture describes it. Man has a sin nature.
But when we say silly things like becoming a Christian is simply an act of the human will, it indicates that we have not seriously considered the impact of sin. Most modern Christians see sin as simply an imperfection. They give it a wink and a nob and proceed to claim to love Jesus and walk about in their sin as if this is an acceptable way for Christians to carry on.
When we hold to views of cheap grace and easy believism, we are denying the essence of what it means to have a sin nature. We are belittling sin. And when we belittle sin, we diminish the holiness of God. And when we belittle sin and diminish the holiness of God, we nullify grace. Grace is no longer a big deal any more. This thinking fails to understand God's revelation of Himself and His attitude toward sin.
An apologetic example will provide a good illustration for what I mean. William Lane Craig has said that if God sentences the American Indian to hell even though that Indian worshipped the Great Spirit and lived according to his internal conscience, that it would make God a monster. I would suggest that the only way Craig could ever make such a claim is by holding to a view of God that is fundamentally different from the one revealed in Scripture. If we are all saved by grace alone, then that ipso facto means that none of us deserve to be saved. This would include the American Indian. It does not matter if he heard the gospel or not. He has certainly sinned against God and such an act demands divine punishment. If the Indian deserves a chance, then it is no longer grace by which we are saved and now God is placed under a moral obligation to at least give us all a chance. Salvation is no longer grace, but a requirement for God. If Craig is correct, the same would have to be true for the Muslim or Jew or Buddhist or "fill in the blank." If a Muslim has never heard the gospel and she is worshipping Allah, and living according to her conscience, God would be a monster to subject her to divine wrath. In other words, according to Craig, idolaters of every stripe are likely to be in heaven even though they worshipped a false god.
You can use whatever illustration you desire for this exercise. Christianity is not a collection of a bunch of people who figured it out intellectually, and made a decision of the will to follow the sayings of Jesus Christ. There are no naturalistic explanations for the phenomenon that is the Christian Church. Mankind fell headlong into sin. As a result he received his just due: the curse of God. This curse separated him from His Creator. It resulted in a corruption of his will, his emotion, and his intellect. Every square inch of man's being became hostile to God. While this hostility and corruption is not at its highest degree, it is nevertheless extensive. Man is now walking in chains, bound to sin, blind, hostile, ignorant, and enemy of God, filled with all sorts of wicked and depraved thoughts. This describes us all. To cap it off, we are completely powerless to change this condition. There is NOTHING we can do and NOTHING we are willing to do to change the state of affairs. Man is not only blind, ignorant and wicked, and incapable of changing these things; man loves his blindness, wickedness, and ignorance. Man is in prison, held captive by sin. And he loves his prison.
The only hope for this desperate condition is the Christ event. To deny this condition flattens the amazing story of redemption we see in Scripture. Man, in his sinful condition cannot help but pretend that it isn't grace that is so amazing, it is himself.
To deny the sinful condition is to deny not only the gospel, it is to deny any need for the gospel. Christ becomes a good moral example and little more. For men who make such assertions, John says the truth is not in them and they are engaging in self-deception.