“For whoever is not against us is for us.” This is said in the opposite way (and perhaps more clearly) in Matthew 12:30: “Whoever is not with me is against me, and whoever does not gather with me, scatters.” Although they are worded differently, both have the same message and neither statement allows for a neutral ground. There is no neutrality in the spiritual battle: we are either for or against God.
Both ways of saying that people were either for you or against you were attested in the ancient world, and Jesus’ disciples were no doubt familiar with the concepts. The world in ancient times was very tough and there were many situations in which neutrality was not acceptable and people had to choose which side they were on. When the Roman philosopher Cicero (106-43 BC) defended the people of Pompeii to Caesar, he quoted Caesar’s own words back to him, saying to Caesar, “Let that maxim of yours, which won you your victory, hold good. For we have often heard you say that, while we considered all who were not with us as our enemies, you considered all who were not against you as your friends” (quoted in The New International Commentary on the New Testament: Mark by W. Lane)
We are all either “for” or “against” God and Jesus. If we are not “against” him then we are for him. If we are not “with” him then we are against him. Someone might say, “Well, I am not against him, but I am not ‘for’ him either.” That statement only shows an ignorance of the spiritual battle and the reality behind the spiritual battle. God created the world, and us, and He demands our allegiance. Someone who is unwilling to recognize God to the point of getting saved is an enemy of God and will end up in Gehenna. Someone who recognizes God to the point of getting saved is part of the Kingdom of God. There is no place where “neutral people” go on the Day of Judgment. The sheep go into the Kingdom, the goats into the Lake of Fire. Being unwilling to commit to being “for” or “against” God is actually part of the Devil’s plot to steal, kill, and destroy (John 10:10). Especially to our modern ears, not being for or against something sounds so reasonable and good-natured that it is easy to think that God must be some kind of Ogre for demanding that we believe in Him. But in the End we will not be able to sweep under the rug the fact that He is our creator, and He created us for a purpose; a purpose that is intertwined with His own purposes, and if we do not want to support Him, then we are in fact against Him.
There is a story about a man who was sitting on a fence, with the Devil on one side and God on the other. God and the Devil were both trying to get the man to come down off the fence to their side. The arguments and pleas went on hour after hour, but the man would not make a decision or come down from the fence. At the end of the day God went home to heaven and the Devil said to the man, “OK, come down and come with me.” The man said, “But I am still on the fence; I have not made a decision yet.” The Devil replied, “Come down. You obviously don’t understand. I own the fence.” How true. If you are not “for” God, you are against Him.