God created man with a pure body, but something of Satan was received into man's body and man's body changed in nature. It was transmuted into the flesh. In other words, it was corrupted. Man's body, by being corrupted in man's fall, became flesh, full of lust. In Romans 7:18a Paul said, "For I know that in me, that is, in my flesh, nothing good dwells." The flesh is the corrupted body.
Through man's eating the tree of knowledge, Satan entered into man and became the very sin within man. To see this point we need to read Romans 7:14b, 17, and 20. In verse 20 Paul said, "But if what I do not will, this I do, it is no longer I that work it out but sin that dwells in me." Romans 7, especially in verses 8, 11, 17, and 20, indicates that sin is a person, the embodiment of Satan, and is living and acting within us. Sin is a personification of Satan. Actually speaking, the sin within us is Satan. At least we can say that the sinful nature within man is the nature of Satan. The sin within man refers to his inward sinful nature. This inward sin is just Satan himself indwelling our corrupted body, that is, our flesh.
In Romans 7:21 Paul says, "I find then the law with me who wills to do the good, that is, the evil is present with me." When Satan entered into man, Satan became sin within man. When this sin acts in man, this sin becomes "the evil." The evil is the action of sin. When sin acts in us, it becomes the evil.
The real significance of man's fall is that man received Satan's thought, feeling, and will, Satan's being, Satan's personality, into the inward parts of his soul. Thus, man's soul became one with Satan's person through man's fall. Also, Satan as sin got into man's body, making man's body the flesh. Satan indwells this flesh as sin, as fallen man's sinful nature. When this sin acts, it is the very evil that moves and acts within man. Through the fall, man's being, man's personality, man's soul, has been filled with Satan's being, with Satan's personality. Satan dwells in man's body as sin, which acts as the evil. Thus, through man's fall, man became one with Satan. This is the reason that the children of men eventually are called in the Bible the children of the devil (1 John 3:10). Fallen man became one with Satan in his personality and in his nature. Man and Satan are one, so the children of men actually are the children of the devil. This is the focus and the real significance of man's fall. We need to point out that man's fall was a receiving of Satan's being into man's being and a taking of Satan himself into man's body, corrupting man's body and transmuting it into the flesh.
Man's fall also resulted in the deadening of his spirit (Eph. 2:1, 5a). Ephesians 2:1 says that in our fallen state we were dead in our offenses and sins. This means that our spirit was deadened. To be deadened is to have the functions damaged. The deadness of our spirit pervaded our entire being and caused us to lose the function that enabled us to contact God. Thus, through the fall, man's soul became one with Satan's person, man's body became Satan's dwelling place, and man's spirit was deadened.