In one word – SIN!
Adam and Eve disobeyed God and by their disobedience they plunged themselves and all those who come after into sin, with a propensity to do likewise (Genesis 3, Romans 5:12-21).
Another word – SATAN!
Before Adam one of the angels fell, Lucifer, was cast out of heaven and began his reign of terror against God and those who follow Him (Isaiah 14:12-17, Ezekiel 28:1-19).
Yet, another word – FLESH!
All of us, even those born again, have some flesh! The Greek word is “sarx” meaning those areas of our life that are not yet submitted to the Lordship of Jesus Christ. And that flesh, when yielded to, has the ability to act contrary to the nature of Christ within (Galatians 5:13-15, 16-21).
Jesus came and lived as a person perfectly obedient to the Father and qualified to be our Savior, died for and as us. All of our sin was put upon Him. He brought forgiveness and eternal life to us. We are recipients of His grace (Ephesians 1:7, 2:8-10).
To win the battle over the flesh, we need to recognize that it was crucified with Jesus (Romans 6:6-12, Galatians 2:20, 5:24, 6:14).
CONSISTENTLY LIVE IN THE SPIRIT OF GOD
Now that we are born again, we are instructed to stand fast in the liberty given to us by the work of Jesus on the Cross and Resurrection (Galatians 5:1). We are instructed to walk in the Spirit and we will not fulfill the lust of the flesh (Galatians 5:16-18, 25). Our lives will either reveal the fruit of the Spirit or the works of the flesh, determined by which one we yield to.