Since Satan can use preachers, how do we know if that person is doing Satan’s work or God’s work?
I would direct your attention to Matthew 7:15-20 which reads, “Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. YOU WILL KNOW THEM BY THEIR FRUITS. Do men gather grapes from thornbushes or figs from thistles? Even so, EVERY GOOD TREE BEARS GOOD FRUIT, BUT A BAD TREE BEARS BAD FRUIT. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Therefore BY THEIR FRUITS YOU WILL KNOW THEM” (NKJV). The Lord Jesus wanted His true followers to KNOW if someone was a false prophet (one being “used by Satan”) and He told them clearly and emphatically “You will know them by their fruits.” What exactly did He mean by that? We have the answer in verse 21, “Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but HE WHO DOES THE WILL OF MY FATHER IN HEAVEN.” In other words, THEIR ACTIONS will reveal if they are doing God’s work or Satan’s work. They may “put on a good show” by pretending to be true believers, but in time they will be manifested as unbelievers who live to do “their own will” instead of living to do “God’s will.”
The Lord went on to say in verses 22-23, “Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’” A prime example of this is seen in Judas Iscariot who kept company with true believers and even did many things in the name of Jesus, but all along he was a thief and eventually he betrayed the Lord for 30 pieces of silver. His ACTIONS betrayed himself and all knew that he was “a bad tree bearing bad fruit.” There are preachers today who claim to be Christian and they may perform many acts of “service,” but like Judas, they are unsaved and in time THEIR ACTIONS will reveal that they are doing Satan’s work and not God’s.
We would also cite 1st John 4:1-3, “Beloved, do not believe every spirit, but TEST THE SPIRITS, WHETHER THEY ARE OF GOD; because many false prophets have gone out into the world. BY THIS YOU KNOW the Spirt of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of Antichrist, which you have heard was coming, and is now already in the world.” We have seen that we can know a false prophet by THEIR ACTIONS; here we learn that we can know them by THEIR DOCTRINE. A true believer will gladly confess “that Jesus Christ has come in the flesh,” which means they believe in the Incarnation of the Son of God. The unbeliever will refuse to admit this truth. They may say “Jesus was a true prophet, a good teacher, and a miracle worker,” but they will NOT admit that He was “God manifest in the flesh.” Jehovah’s Witnesses, Mormons, and many other cults fall into this category, but there may even be those who are among true Christians who fail to acknowledge this blessed truth. If so, they are “not of God” and it is said of them “this is the spirit of Antichrist.” They are, in truth, OPPOSED to Christ. I believe the Lord will say to them also in the Day of Judgment, “I never knew you; depart from Me, you who practice lawlessness.” (266.9) (DO)