The sinful nature of man
The thing we must first focus on (as always) is the account in Genesis 2 and 3. Here, the author (presumably Moses), shows us how God prepared man and a place for him. He made man with free will and gave him dominion over creation (animals/plants/etc.). In the Garden in Eden man apparently walks with God. This is also mentioned later in the Scripture when Enoch walks with God. This must be interpreted to be as much as a lifestyle (faith) as also an action. Walking with God is what man was intended to do. Not only man did this but so did woman. Life was joyful. Then the serpent came. (dun dun dun). The serpent provided an extra temptation to the fruit with his words to the woman. The sin here is the mans/womans desire to judge what is good and bad, to attain the knowledge that God possesed. This was a great sin indeed, for Man (from now on I will refer to man and woman with the word Man) does not know what is best for Man, only God knows what is best for Man. Therefore, whenever Man has his idea of what is best he is also sinning against God.
One of the main themes in this book is that The Lord will provide. The Lord created the man and naturally knows what is best for him. The Lord provided man with the most appropriate mate (Gen 2:22-24). The Lord, after Man sinned, provided man with clothes (Gen. 3:21). The Lord provided for Abraham (Gen 22:8,13). The Lord provides! Jesus rebuffed this himself, Look at the birds, flowers, etc. they dont sow, harvest or worry and yet are provided for (Matt 6:25-33). Mans desire to choose for himself and provide for himself, denies God and doesnt give Him glory.
Choosing what is right and wrong is an inherent trait in Man and Man is cursed for it; dust you are and dust you shall become. We are condemned a death, something that wasnt supposed to happen in the Garden in Eden. Man was expelled and doomed to die. We, therefore, have this burden on us, the burden of death, for all men born under Adam, the natural way, are subject to his failing.
The tendencies of this Will of Man, inherited from our line, are that of the flesh, so talked in the New Testament. For our will is to please our flesh (Gal 5:19-21), and that is not the will of the Spirit, i.e. God (Gal 5:22-23). We only have a Carnal Mind, a mind that deals with the flesh and wants for the profit of itself. Paul taught this in his epistle to the Galatians where he continually stated that the spirit is opposite the flesh. Jesus taught, he who wants to preserve his life, will lose it, and those who lose it will keep it (Matt 10:39, Matt 16:25, Mark 8:35, Luke 9:24, Luke 17:33). One can think of this as, he who does his own will, will lose his life, but the one who does the will of God, will live eternally and be forever joyful in His presence.
Man is unable to break his carnal thoughts. He is obsessed with choosing the right and wrong things, with judging what is best for him. Until man can let go, receive the Holy Spirit and rely on God, he cannot escape his carnal thought. Therefore, we must Walk with God by virtue of the Holy Spirit, if not we will surely perish. The fruits of men fail, but the fruits of God abound always. Proverbs 3:5 says it best Trust in the Lord with all your Heart!
-James
(PS this can be further discussed as to its roots with respect to infant baptism, etc.)