2 By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, 3 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 the Antichrist, which you have heard was coming, and is now already in the world.
4 You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world. 5 They are of the world. Therefore they speak as of the world, and the world hears them.
Matthew Henry's Commentary on 1 John 4:2-5
Commentary on 1 John 4:1-6
(Read 1 John 4:1-6)
Christians who are well acquainted with the Scriptures, may, in humble dependence on Divine teaching, discern those who set forth doctrines according to the apostles, and those who contradict them. The sum of revealed religion is in the doctrine concerning Christ, his person and office. The false teachers spake of the world according to its maxims and tastes, so as not to offend carnal men. The world approved them, they made rapid progress, and had many followers such as themselves; the world will love its own, and its own will love it. The true doctrine as to the Saviour's person, as leading men from the world to God, is a mark of the spirit of truth in opposition to the spirit of error. The more pure and holy any doctrine is, the more likely to be of God; nor can we by any other rules try the spirits whether they are of God or not. And what wonder is it, that people of a worldly spirit should cleave to those who are like themselves, and suit their schemes and discourses to their corrupt taste?