Even if we do not sin, our sinful flesh has iniquities that are impure. We are, in the most literal sense, sinful by nature as humans because our outward man is of the flesh that sins.

The "thorn in the flesh" is the will of the flesh. It is a thing, not a who.

There is a will of the flesh (a thorn) in every person, but how is it a "messenger of Satan"? From Websters 1828 dictionary,

• "Messenger (messager)" is: 1. One who bears a message or an errand; the bearer of a verbal or written communication, notice or invitation from one person to another, or to a public body; one who conveys dispatches from one prince or court to another. 2. A harbinger; a forerunner; he or that which foreshows.

The will of the flesh is a rebellious desire inside the flesh of all humans, including Jesus, and God knows all about it. 

• For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin. Hebrews 4:15.

God uses our weakness (infirmities) to propel us to Him through our reliance on the Spirit as the inner strength living in us as we live "inside out" by the Spirit.

The apostle Paul (also called Saul from Tarsus before his ministry) was chosen by the Lord Jesus to spread the Word of God. (See, Acts 9:11-16.). Paul wrote about 2/3 of the New Testament. He spoke of our "thorn in the flesh" this way:

• For this thing [the thorn in the flesh] I besought the Lord thrice, that it might depart from me. And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness... 2 Corinthians 12:8-9.

That's why we glory in our infirmities and take pleasure in them for Christ's sake because we know when we are weak, the Spirit of God makes us strong! 

• ...Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong. 2 Corinthians 12:9-10.

Let's be clear; we do not continue in sin for grace to abound. God forbid!

• What shall we say then? Shall we continue in sin, that grace may abound? God forbid. How shall we, that are dead to sin, live any longer therein? Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. Romans 6:1-4.

We can't. It is the spur of our iniquities that are stuck in us making us sinful by nature. Nobody can control the desires of their iniquities, see Romans 7:14-20. The born again children of God turn to the Spirit (the Word and the Holy Ghost as the "comforters" or helpers) to help us control our flesh, but here again, we must listen and obey to mortify the sinful iniquities of the flesh.

• For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. Romans 8:13.

By contrast, whoever is not born again of the Spirit of God is left to control their own flesh as a child of the world, and they shall fail because of their infirmities stemming from their iniquities. The Word says so.

As we said earlier, the will of man associated with the soul of every person has one most important choice: live by the will of the Spirit or live by the will of the flesh. When we choose the Spirit, the Spirit is the warrior in us. Whenever we choose the flesh, the flesh is the warrior, and sin is the result because that's what the flesh is, sinful.

• Dearly beloved, I beseech you as strangers and pilgrims, abstain from fleshly lusts, which war against the soul; 1 Peter 2:11.
• For though we walk in the flesh, we do not war after the flesh: (For the weapons of our warfare are not carnal, but mighty through Go to the pulling down of strong holds;) 2 Corinthians 10:3-4.

For the born again, we find our strength, the Spirit, inside our weakness (inside our flesh) and live "inside-out" by the Spirit.  Whoever isn't born again of the Spirit (saved) is condemned already and shall continue in sin living "outside-in" as friend of the world and enemy of God. The Word says that too.  

Our iniquities generate our fleshly infirmities (weaknesses) and the devil knows it. Whenever we think the Spirit, or God's sovereign will, is insufficient to satisfy us, we demonstrate our infirmities and desire a fleshly alternative to God or His ways because our body and mind lust to satisfy the iniquitous natural man, and we find a place of temptation from the enemy.

• Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak. Matthew 26:41.
• Watch ye and pray, lest ye enter into temptation. The spirit truly is ready, but the flesh is weak. Mark 14:38.

The resultant behavior of our flesh is a form of temptation (a signaler of our iniquities), but it lacks the opportunity to sin. 

• And my temptation which was in my flesh ye despised not, nor rejected;... Galatians 4:14.

The devil provides the enticing opportunity as the temptation to sin. The devil's temptations (enticements) to sin are designed to satisfy our iniquities, prey on our infirmities (our flesh), because the will of our flesh signals our favorite flavors of sin as weaknesses to the enemy.  

• Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man: But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. Do not err, my beloved brethren. James 1:13-16.

Broad is the way for the unsaved who befriend the world for satisfaction of their iniquities, for they seek the bogus religions to make place for their infirmities and appease them in avoidance of repentance therefrom.

• For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears; And they shall turn away their ears from the truth, and shall be turned unto fables. 2 Timothy 4: 3-4.