What was Paul's "thorn in the flesh?"
The messenger of Satan was absolutely a demonic persecution toward Paul. If you read ch. 11:23-28, Paul includes a list of all his trials. They were all caused by Satan's messenger (a demon) who stirred up people everywhere Paul went in order to hinder the gospel and keep Paul from being exalted too highly by the people (this would have given him more religious influence in the region, and more Christian converts would have been snatched from the devil, so Satan wanted to stop him). The scripture never says that God gave Paul the thorn to keep his pride in check and develop his character. Instead, it says that the messenger came from Satan. The word 'messenger' is actually the Greek word "aggelos", which is usually translated as "angel" everywhere else in the Bible (even in ch. 11:14). So if the messenger came from Satan, it would be some demon or demonic influence to hinder Paul's ministry.
If Paul actually had a pride problem (and he didn't), the devil certainly wouldn't have tried to stop him. The devil would have rejoiced that Paul was in pride because 'pride goes before fall' and a 'haughty spirit before destruction.'
The notion that this is God's passive will and that Satan is God's controlled agent to help character development is wrong. God doesn't use Satan to help develop the Christian's character. God has given his Holy Spirit and His Word to help perfect the Christian's character, and He doesn't use the devil. This erroneous teaching that God and Satan are somehow partners in the divine plan has caused much confusion. James ch. 1 says not to error and "let no man say" that God tests or tempts anyone with any sort of evil. Temptation comes from Satan. Only good things come from God. The correct interpretation is that Satan has the ability to afflict many people with many things. Just as God had to sit by and watch Satan tempt Eve and Adam in the beginning, God also has to sit by and let Satan mess with people now. But now, since Christ arose, we can overcome all the works of devil, including sin, sickness, and oppression, by faith in God and the authority of Jesus' name. The one form of trial that God does not promise us deliverance from is ?persecution for righteousness' sake'. Jesus promised that we would also be persecuted for preaching the gospel and for righteousness, just as He was. So, God has already made a standing decision that persecution would be allowed since "the servant is not greater than his master.". Persecution is what happened to Paul.
Some have said that the 'thorn in his flesh' was some sort of sickness and that God is the one who gave it, but the scripture clearly defines the thorn as ?the messenger from Satan'. The term 'thorn in the flesh' is an old testament reference to people who are a constant problem. It is very similar to our colloquial saying of "that person is just a pain in the neck", "or a thorn in my side". And God gives us grace (ability and power and strength) to endure through hardship and overcome that hardship. Notice that Paul certainly didn't die. And Paul never mentioned any sort of disease that afflicted him. It was only persecution. The same persecution that Jesus faced.