1 John 4:1-6 Beloved, BELIEVE NOT every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. 2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: 3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world. 4 Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world. 5 They are of the world: therefore speak they of the world, and the world heareth them. 6 We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error.
In John’s day there were many people and false prophets that he called “antichrists” (against Christ, opposing Christ, in place of Christ) whose speaking was inspired from another spirit, the spirit of error. Just because these antichrists had a following, had people hearing them, listening to them, did not make them of God or their teachings valid. Their words and teachings needed tested. They were being exhorted by the Apostle to stop believing every spirit, every teaching said to be of the Spirit of God, or every so called inspired utterance. They were being cautioned about being gullible and to not believe and put their trust in every inspired speaker. They needed to be tested for what spirit they were of, by their belief in the Incarnation of Jesus Christ, or that Jesus had come in the flesh, and that Jesus is God’s Son.
These believers were of God and had overcome these false teachers, prophets, antichrist’s by the Greater One in them, the Holy Spirit. They had the unction from the Holy One or anointing of the Spirit in them to help them know who was teaching them the truth (1 Jn. 2:20, 27). Keep in mind that they didn’t have the completed NT to test what was taught but their teaching was the Apostles teaching or doctrine (Acts 2:42). The people that were of God, that know God, listen to the Apostles, and are listening to the spirit of truth. People, who were not of God, were of the world, and they listen to the antichrists, and the spirit of error.