Paul speaks very clearly in Galatians 5 about our battle against the enemy within ... ourselves. All too often we look to others, or even demonic spiritual forces, to be the source, the cause (and usually the blame) for our sins. Although sometimes their can be external forces, ultimately all responsibility lies at our doorstep. Our greatest battle is truly against our own flesh, the sin nature that resides in every one of us. To deny or to minimize its strong pull on our will is to succumb to the lie that we are, in ourselves, good. Man isn't a sinner because he sins. Man sins because He is a sinner. We must remember that we are all born in a fallen state and with a sin nature. Two verses that remind us of the evil within are: "The heart is deceitful above all things, and desperately wicked: who can know it?" (Jeremiah 17:9).
"As it is written, There is none righteous, no, not one" (Romans 3:10).
Keep in mind the context of these verses are about unregenerate man. When God saves us He takes away the hard heart and gives us a new heart. But, we must remember that while our sins are forgiven and while we are also given eternal life and the gift of the Holy Spirit, even with the new heart of regeneration we continue to sin. And, as it has been since the Garden, we are all tempted by these three things: (1) the lust of the flesh, (2) the lust of the eyes, and (3) the pride of life (1 John 2:16).