The following is an excerpt from The MacArthur New Testament Commentary on 2 Corinthians 12.

Because of the surpassing greatness of the revelations, for this reason, to keep me from exalting myself, there was given me a thorn in the flesh, a messenger of Satan to torment me—to keep me from exalting myself! (12:7)

The evidence of Paul’s success in ministry was the power of the gospel to transform lives that led to the churches he founded and built up. They were a monument to his faithfulness and to God’s power working through him. To see any of those churches being led astray by false teachers was a painful, humbling experience for Paul, yet one that he needed. Twice in verse 7 he emphasized that God allowed his thorn in the flesh to keep him from exalting himself. Though he was the noblest Christian of all, Paul was not impervious to the normal struggles of life. Certainly, because of the surpassing greatness of the revelations he had experienced (Acts records six visions apart from the one related in this passage; he also received the gospel he preached by revelation [Gal. 1:11–12; cf. Eph. 3:3]), pride was a constant temptation. Therefore, to keep him humble, Paul was given … a thorn in the flesh, a messenger of Satan to torment him.

The identity of that thorn in the flesh has been much debated. Paul did not describe it in detail, indicating that the Corinthians knew what it was. Most commentators assume it was a physical ailment, such as migraines, ophthalmia, malaria, epilepsy, gallstones, gout, rheumatism, an intestinal disorder, or even a speech impediment. That such a wide range of possibilities has been put forth indicates a lack of specific support in Scripture for any of them. (Even if Paul’s words in Gal. 6:11, “See with what large letters I am writing to you with my own hand,” indicate an eye problem, there is nothing in that verse that connects it with the apostle’s thorn in the flesh.) Skolops (thorn) could be better translated “stake,” graphically indicating the intensity of the suffering it caused Paul; it was not a small thorn but a large stake.

The apostle acknowledged that the thorn in the flesh ultimately was given to him from God. (See Job 1 and 2, where God permitted Satan to afflict Job for His own purposes; cf. Gen. 50:20.) The phrase in the flesh could also be rendered “for” or “because of the flesh.” Flesh should be understood here not in a physical sense, but in a moral sense as a reference to Paul’s unredeemed humanness (cf. Rom. 6:19; 7:5, 18, 25; 8:4–9). The Lord allowed Paul’s intense suffering to impale his otherwise proud flesh; to humble the one who had so many revelations.

It is best to understand Paul’s thorn as a demonic messenger of Satan sent to torment him by using the deceivers to seduce the Corinthians into a rebellion against him. At least four lines of evidence support that interpretation. First, in the overwhelming majority of its uses in the New Testament (including every other occurrence in Paul’s writings), angelos (messenger) refers to angels. An angel sent from Satan would, of course, be a demon. Second, the verb translated torment always refers to harsh treatment from someone (Matt. 26:67; Mark 14:65; 1 Cor. 4:11; 1 Peter 2:20). Third, the Old Testament sometimes refers metaphorically to opponents as thorns (e.g., Num. 33:55; Josh. 23:13; Judg. 2:3; Ezek. 28:24). Finally, the verb translated leave in verse 8 is always used in the New Testament to speak of someone departing. Likely, then, the demonic messenger was tormenting Paul by being the indwelling spirit in the leading false apostle (cf. 2 Cor. 11:13–15; 1 Tim. 4:1). Again, this is consistent with Paul’s testimony that his severest suffering came from his concern for the church (11:28–29).

Satan’s assault on Paul did not take place outside of God’s will. God is sovereign over all of His creation and will use even the forces of the kingdom of darkness to accomplish His righteous purposes (cf. Num. 22:2–24:25; 1 Kings 22:19–23; Luke 22:31–32). Paul was critical to God’s redemptive plan, and He would keep him humble by whatever means necessary, including using a demon. If this seems unusual, remember the accounts of Job (Job 1:6–12; 2:1–7) and Peter (Luke 22:31). In both cases God allowed Satan to bring devastating suffering on His saints to achieve their greater usefulness. This is a good reminder of the foolishness of those who try to tell Satan and demons what to do and where to go. If we could command demons, we might thwart the purposes of God with our faulty assumptions.