Demons or Flesh?
"But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. 18 But if you are led by the Spirit, you are not under the law. 19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality, 20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, 21 envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. 22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires.
25 If we live by the Spirit, let us also keep in step with the Spirit. 26 Let us not become conceited, provoking one another, envying one another." (Galatians 5:16-26 ESV)
Does a person commit a sin--that is, disobey God’s orders--because of demons or because of their flesh (sometimes translated "sinful nature?") Study also Romans 6:5ff; Ephesians 4:17-24; Colossians 3:5-10.
The two are very interconnected, working together, so it’s often a “both-and-thing” rather than an “either-or-thing.”
"Then a demon-oppressed man who was blind and mute was brought to him, and he healed him, so that the man spoke and saw. 23 And all the people were amazed, and said, 'Can this be the Son of David?'24 But when the Pharisees heard it, they said, 'It is only by Beelzebul, the prince of demons, that this man casts out demons.'" (Matthew 12:22-24 ESV)
What is the meaning of “Beelzebul?” It has been translated “Lord of the flies,” referring to Satan. Satan, called “the god of this world” in 2 Corinthians 4:4, heads a kingdom of darkness with a hierarchy of evil spirits consisting of fallen angels, principalities, and/or demons. C.f. Isaiah 24:21-23; Ephesians 2:1-3; 6:12. Revelation 9:1ff; 12:7-9. Demons operate much like flies, as the name “Beelzebul” insinuates.
What attracts flies? Dead, rotting flesh. “Wherever the corpse is, there the vultures will gather” (Matthew 24:28 ESV). Similarly, if one struggles with a particular sin--a work of the "flesh," or "the old man"--a corresponding demon could be involved in that process. Like flies are drawn to the stench of rotting flesh, demons are drawn out from their pit and drawn to people according to their sinful desires. Just because someone falls into sin on occasion, that doesn't necessary mean they have a demon, but a demon may have entered if that sin becomes a chronic addiction or if nothing ever seems to go right in the person's life. Other similarities between flies and demons include their numerous amounts (numbering in the millions or even billions), and their defiling characteristics.
How then, does one repent (or turn from) a particular sin and gain victory over it?
"You adulterous people! Do you not know that friendship with the world is enmity with God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God. 5 Or do you suppose it is to no purpose that the Scripture says, 'He yearns jealously over the spirit that he has made to dwell in us'? 6 But he gives more grace. Therefore it says, 'God opposes the proud but gives grace to the humble.' 7 Submit yourselves therefore to God. Resist the devil, and he will flee from you. 8 Draw near to God, and he will draw near to you. Cleanse your hands, you sinners, and purify your hearts, you double-minded. 9 Be wretched and mourn and weep. Let your laughter be turned to mourning and your joy to gloom. 10 Humble yourselves before the Lord, and he will exalt you."
(James 4:4-10 ESV)
According to verse 7, we resist the devil (a term including demons) by submitting to God. We oppose Satan by drawing near to God, humbling ourselves before Him, and letting Him have His way with us as we commit to follow His orders, to love His ways, and to delight in His Word. Since sin and demons are so interconnected as to be inseparable, then repentance, agreeing with God, submitting to God, and resisting the devil and his temptation are likewise inseparable. Surrendering to God as His servant, repenting of one's sins, receiving His forgiveness and love, being filled with the Holy Spirit, praising and worshiping Jesus, meditating on and obeying the Word of God, and casting out demons are all part of what is means to be Jesus' disciple.
If you are a follower of Jesus, then do all and each of these as your Guide and Counselor, the Holy Spirit, directs. See John 14-16.
If you have not yet begun a relationship with God as His child and servant, after which He comes to live inside you to empower you to live Jesus' life, then see my article "Would You Like to Kick Satan's Butt Personally?"