John told us not to believe every spirit. We should discern if the spirit is from God or not. This meas that there are those who are preaching messages that are spirit led but not Holy Spirit led. It means that these "spiritual" messages will sound good enough to be believed. They will have a power that is beyond the ability of the preacher to deliver them. They will be from the devil. Evidently, many people will be led away by these preachers.
You do not test them to see if they are false. You test to see if they are from God. You examine anything by putting it up against a known truth. You don't look at a board and say, "I want to prove that this board isn't eight foot long." Instead, you take the board and measure it using a tape measure which is a known true standard.
This must be constant for all the preaching you hear. You cannot build a house using "supposed eight foot boards." Your house will not be square or strong if you are not constantly measuring to see if each board is true to the measure it is supposed to be.
John says the standard is whether or not the person preaching confesses that Jesus has come in the flesh. The word "confess" means "say the same words." The spirit leading this preacher must say the same thing that God says about His Son. This preacher must say that Jesus is the only way to salvation. This preacher must say that Jesus has died for our sins. This means this preacher must say that we are sinners in need of God's grace which is found in Jesus, His Son.
Some have said they have confessed Jesus simply because they say they believe in Him. That doesn't grasp the full meaning of what this scripture is saying. It is like saying the person is an American because he can sing all the words to "God Bless America."
This person "confesses" through their continuing understanding of the words of Jesus as recorded in the Bible. They may make mistakes but they will not be spirit led when they do so- neither Holy Spirit led nor other spirit led. Their confession should come through their own growth in the faith. They will be able to give more Holy Spirit led revelation as they have grown spiritually. They will be better able to discern the Bible, to interpret circumstances for God's will, and discern the truth others are professing when they have grown in the Lord.
You must always be aware that there are those who will lead people astray. They may have huge churches. (Why wouldn't the devil want to build a huge church if it would lead people away from the Lord?) They may be good looking and be able to preach with words that seem to come beyond this world. (As indeed they do.) Many will be taken away from following the Lord because of them.
Don't be one of them.
I John 4:1-3 (NASB)
Beloved, do not believe every spirit, but test the spirits to see whether they are from God; because many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; and every spirit that does not confess Jesus is not from God; and this is the spirit of the antichrist, of which you have heard that is coming, and now it is already in the world.