In effort to keep it simple, it is someone influenced through, their adamic nature, by Satan, to undermine God's authority with their own. A spirit of Jezebel is simply a way of describing someone that allows themselves to be used by the devil, same as Peter when he tried to discourage Jesus from going to Jerusalem to die.
But unlike Peter this individual is not thinking of the betterment of God's Kingdom they are manipulated by the enemy in an effort to undermine God's authority in His church. This was a characteristic of Jezebel, the wife of King Ahab, who we see in 1 Kings. Jezebel was the real power behind the throne. When Jezebel came on the scene one of the first things we see is her killing the prophets of God and installing her false prophets. When the prophet Elijah defeated the prophets of Baal it was Jezebel that was determined to kill Elijah.
It is not necessarily someone possessed by an evil spirit but rather someone that is being influenced by an ungodly spirit. This is something not that uncommon in the church today. Especially given the churches failure to exercise biblical authority most of the time.
Often times one that is being manipulated by such a spirit will be viewed as someone that might seem helpful. But this individual will lure others away from the things of God to establish their own following, and eventually using manipulative tactics to undermine God's established authority.
Often times the individual, if confronted, will seek to be victimized by those in authority in church, thus making themselves a victim and garnering the sympathy of the young believers that have little knowledge of the Scriptures, and who struggle with authority themselves.
The person that exercises a spirit of Jezebel can be either male or female. But often the case tends to be a woman, as it was in the garden. Sadly this will often be the case in the church due to a lack of spiritual authority in the home. You will find women easily influenced because there is a failure of men in the church to be the spiritual leader God calls them to be.
So, as the Bible tells us we must keep in mind, we do not wrestle with flesh and blood, but principalities and powers. Our battle is not with people, but with the spirits that try to udermine God's authority in His church, and the only way to defeat those spirits is the same way Jesus taught us by using the Word of God. IT IS WRITTEN.
As long as the Church stands upon the Word of God there is no power of Hell that can pull Her down.
