The thorn in the flesh was given to Paul that he might not be puffed up. It was some infirmity which rendered him despicable in preaching (Gal. 4:13-14); it was a counterpoise to the glory he had tasted. It does not follow that we should necessarily have the same thorn, for God gives to each of us the very thorn which is suitable for us.
It is Satan whom God uses against the flesh: the flesh is seen in four different circumstances.
1. Before conversion, it is under Satan's power, the conscience being hardened. As Judas, who loved money, and was a thief. When he had taken the sop, Satan entered into him to lead him on to unbridled iniquity, and to give him over to despair when he saw the result of his crime.
2. Before conversion, the flesh acts, and Satan presents the temptation.
3. After conversion, the flesh is still there; the Holy Ghost (who is the seal of redemption), has not yet fully accomplished his work in us; we are weak like Peter. Peter is contrasted with Christ in almost every circumstance, even the most amiable. Before the transfiguration, when Jesus tells of His approaching sufferings, Peter answers with love, but yet according to the flesh, and Jesus reproaches him thus, "Get thee behind me, Satan" (Matt. 16:20, 23). In the Apostle Peter, the flesh was still under the dominion of Satan.
4. Satan desires to sift us like wheat, by means of the flesh. Jesus warns His disciples of this, and prays especially for Peter, in whom the flesh was strong, for he put himself forward on all occasions.
Whenever a Christian acts in the flesh, all the weight of his piety goes to sanction and authorise his conduct before others: and therefore the effects of the working of the flesh in a believer are far worse than in an unconverted person. Peter, for instance, had carried away all the Jews, including Barnabas, in his dissimulation.
Being caught up to the third heaven does not change the flesh; and the flesh in Paul was ready to say, "You are the only man who has been in the third heaven." But then the messenger of Satan was permitted to buffet him and became thus the instrument of God's goodness in correcting His child. From an old magazine.