Our sinful flesh has a messager (messenger) of the enemy.

Webster's 1828 dictionary (available online) defines "messager" (messenger) as: 

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

• Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. John 1:13.
• For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not. Romans 7:18.
• That he no longer should live the rest of his time in the flesh to the lusts of men, but to the will of God. 1 Peter 4:2.

The will of the flesh is that "thorn" in our flesh that tells us (messages) and the world about the ways we regard the will of God as insufficient for us, and we prefer the rebellious desires inside our flesh that lead to sin. All humans have a will of the flesh, 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 weaknesses (infirmities) to propel us to Him through our reliance on the Spirit, the "inner strength" living in us, as we live "inside out" by the Spirit instead of outside in by the flesh.

• Because the foolishness of God is wiser than men; and the weakness of God is stronger than men. 1 Corinthians 1:25.
• 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 12:10.
• And what shall I more say? for the time would fail me to tell of Gedeon, and of Barak, and of Samson, and of Jephthae; of David also, and Samuel, and of the prophets: Who through faith subdued kingdoms, wrought righteousness, obtained promises, stopped the mouths of lions. Quenched the violence of fire, escaped the edge of the sword, out of weakness were made strong, waxed valiant in fight, turned to flight the armies of the aliens. Hebrews 11:32-34.

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!

Let's be clear though; 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.

Our iniquities generate our fleshly infirmities (weaknesses), and the devil knows them. The enemy has watched us our whole life. It knows who we are and our weaknesses. Whenever we act in such a way that proves God's sovereign will is insufficient to satisfy us, we demonstrate our infirmities by desiring a fleshly alternative to God and His ways.

• If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him. But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed. For let not that man think that he shall receive any thing of the Lord. A double minded man is unstable in all his ways. James 1:5-8.

Double-minded is actually rendered "doule-souled" in the King James New Testament. That means we want two lives. A double-minded ("double-souled") man wants both to live by the will of his flesh and the will of God.the

When we live in satisfaction of our flesh instead of God, we give in to  lusts of our flesh to satisfy the iniquitous natural outward man, and we find a place of temptation from 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.

This resultant behavior of our flesh is a messenger (signaler) of our iniquities, but it lacks the opportunity to sin. The devil provides the enticing opportunity to sin. The devil's temptations (enticements) to sin are designed to satisfy our iniquities and prey on our infirmities (our flesh); as we live out the will of our flesh, our favorite flavors of sin signal our weaknesses to the enemy.  

We can't. Our will of the flesh is the spur of our iniquities that lead us to sin. Sin is in our flesh is exposed by the thorn (will of the flesh). The thorn in the flesh (will of the flesh) is attached to the sin in us and protrudes from us like a spur when we choose to live by the will of the flesh instead of the will of God. 

Nobody can control the will of their flesh without the Spirit. Being born again of the Spirit sets us free from the penalty of our sin and keeps our thorn hidden from the enemy.

• For we know that the law is spiritual: but I am carnal, sold under sin. For that which I do I allow not: for what I would, that do I not; but what I hate, that do I. If then I do that which I would not, I consent unto the law that it is good. Now then it is no more I that do it, but sin that dwelleth in me. For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not. For the good that I would I do not: but the evil which I would not, that I do. Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me. I find then a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man: But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! who shall deliver me from the body of this death? I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin. Romans 7:14-25.

The born again children of God turn to the Spirit through the Word and the Holy Ghost (Spirit of Truth) as the "comforters" (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. 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 (God) or live by the will of the flesh (your self). Remember, there are three and only three wills the Bible speaks of: 1.) Will of the Flesh, 2.) the Will of man, and 3.) the Will of God.

• ...power to become the sons of God, even to them that believe on his name: Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. John 1:12-13.

When we choose the Spirit (God), 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 through the Spirit of God, by the Word (e.g., reciting scriptures as our weapons) and the Holy Ghost (e.g., in prayer with the Lord), as we seek to live "inside-out" by the Holy Spirit dwelling in us.  Whoever isn't born again of God lacks the Spirit and can't overcome the sin in their flesh. They remain condemned and shall continue in sin, living "outside-in" as a friend of the world and an enemy of God. The Word says that too.

• 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.
• And when he was at the place, he said unto them, Pray that ye enter not into temptation. Luke 22:40.
• And said unto them, Why sleep ye? rise and pray, lest ye enter into temptation. Luke 22:46.

By way of example, husbands and wives:

• Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency. 1 Corinthians 7:5.

Married couples, a man and a woman, are sanctified by the Spirit. They can do privately whatever carnal things they choose that suit them both as one flesh to keep each other free from lustful iniquities.