Even if we do not sin, we have iniquities within us and they are impure. We are, in the most literal sense, sinful by nature.

From Websters 1828 dictionary,

• "Infirmity" is: 1. An unsound or unhealthy state of the body; weakness; feebleness.  2. Weakness of mind; failing; fault; foible. 3. Weakness of resolution. 4. Defect; imperfection; weakness.

Thus, weakness of our mind and body (flesh) is an infirmity (defect).

Yes. Our will of the flesh is the "spokesman" of our iniquities. Our iniquities make us infirm, and our propensity to sin results from our infirmities. 

Before we are born again, we must know we are separated from God because of our iniquities (and sin), but the Lord wants to save us from both.

• Behold, the Lord's hand is not shortened, that it cannot save; neither his ear heavy, that it cannot hear: But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear. Isaiah 59:1-2.

From Webster's 1828 Dictionary,

• "Iniquity" is: 1. Injustice; unrighteousness; a deviation from rectitude; 2. Want of rectitude in principle; 3. A particular deviation from rectitude; a sin or crime; wickedness; any act of injustice. 4. Original want of holiness or depravity.
• "Rectitude" is: In morality, rightness of principle or practice; uprightness of mind; exact conformity to truth, or to the rules prescribed for moral conduct, either by divine or human laws. Rectitude of mind is the disposition to act in conformity to any known standard of right, truth or justice; rectitude of conduct is the actual conformity to such standard. Perfect rectitude belongs only to the Supreme Being. The more nearly the rectitude of men approaches to the standard of the divine law, the more exalted and dignified is their character. Want of rectitude is not only sinful, but debasing. 

As babies, we were pure before God. As we grow in this world, whatever we like that is contrary to complete reliance on God and His ways can become iniquitous. Whatever satisfaction we seek from the world as a friend of it proves we are infirm, sinful humans.

Stated another way, our flesh knows what flavors of sin we like most (iniquity). The will of our flesh is our connectivity to the world and the god of the world, the enemy (the devil). Our will of the flesh is like a barb (thorn) the enemy tries to snare us by with our favorite flavors of sin because we are infirm, sinful humans, hence the thorn.

Yes!  Whoever is born of the Spirit receives the cleansing of the Word of God and the renewing (quickening) of the Holy Ghost and is sinless as part of the body of Christ. That's why we have power over all the power of the enemy, and have no excuse for not doing the Father's will.

When we are born of the Spirit (born again, the saved), God does not see us as sinful or iniquitous, even if the devil may. The Father and the devil both see us as part of Jesus (see John 17).

• It is a faithful saying: For if we be dead with him, we shall also live with him: If we suffer, we shall also reign with him: if we deny him, he also will deny us: If we believe not, yet he abideth faithful: he cannot deny himself. 2 Timothy 2:11-13.

The born again of the Spirit are as if we died with Jesus on the cross.

• Therefore 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 6:4.

When we resist the devil, we find the strength in our weak flesh is the Spirit. 

Remember, as the born of the Spirit the weapons of our warfare are not carnal but mighty through God to the pulling down of strongholds, including the strongholds of iniquity in us and the defiant will of our flesh, see 2 Corinthians 10:4. Until we go to heaven, our sinful flesh in this world remains a corruptible sack of iniquity, and the enemy tempts us through our sinful flesh no matter how glorious we think we are or make it with exercise, adornment, pretense, or profession. 

In fact, the born again die daily to the deeds of the flesh and get stronger every day.

• 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.
• I protest by your rejoicing which I have in Christ Jesus our Lord, I die daily. 1 Corinthians 15:31.

We are cleansed by Jesus the Word and renewed by the Holy Ghost through repentance from our iniquities that cause future sin, thereby sealing us until the day of redemption according to His holy Spirit of promise.

• Unto you first God, having raised up his Son Jesus, sent him to bless you, in turning away every one of you from his iniquities. Acts 3:26.
• Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered. Romans 4:7. 
• In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory. Ephesians 1:13-14.

As the born-again children of God, eternally alive by the Spirit, the Father does not remember our iniquities or see our sins because of what Jesus did, and set in motion, from the cross.

• Wash me throughly from mine iniquity, and cleanse me from my sin. Psalm 51:2.
• For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more. Hebrews 8:12.
• And their sins and iniquities will I remember no more. Hebrews 10:17.

We must remain penitent because our infirmities are still in our flesh and remain visible by the will of our flesh like a thorn we cannot remove. Our thorn in the flesh seeks to live in satisfaction of our iniquities and prove our infirmities, causing us to sin (make mistakes) in the flesh. THIS IS YOUR BATTLEFIELD! It is within you.

• Dearly beloved, I beseech you as strangers and pilgrims, abstain from fleshly lusts, which war against the soul; 1 Peter 2:11.

If we sin, we repent. If we sin again, we repent again,...

• In your patience possess ye your souls. Luke 21:19.

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.