In this familiar saying (Lincoln quoted this in his famous "House Divided" speech before the Civil War) there are the answers to three of the more common misconceptions about Christ and his Church. We take them in their common form:
"Can't I be a good person without being a Christian?"
One is tempted to respond, "Why would you want to?" But the answer to the question is simply, "No." Suppose you have some besetting problem, a sin, which dominates your life. By your own efforts, self discipline and whatever aid the world can give you, let us suppose you throw it out and triumph over it. This is good—but our Lord tells you what happens next. Satan is not through with you; if lust will not do, then greed; if not greed, then pride—for the seven spirits here could indeed be the Seven Deadly Sins. Satan's object is to have you "safe" - the word in the original means "at peace" - quiet and comfortable in your life of sin. He will not quit until he has succeeded—or you have found the Strong Man who throws him out, taking his armor (his ability to fight).
"Aren't Satan and Jesus equal and opposite? Really, is there a difference?"
The picture here is quite clear. Jesus is the "strong man" who throws out Satan; Satan does not throw out Jesus. You have to do that if you want Satan back. There is a difference between right and wrong; the victory is already decided.
"I don't really have to make up my mind about this, right? After all, God will forgive me for an honest mistake."
Calvin Coolidge had it right: "Sometimes being open minded is just being too lazy to make up your mind." As verse 23 makes clear, you have already decided. You are either with Christ or against him.
The matter is fairly simple. Can you choose to make an honest mistake? No, you can't. You can make it out of ignorance—but if you're reading this, you're not ignorant any more. You can make it because you're misled (not if this author can help it). But you cannot know about Jesus and then decide, "I'll just make an honest mistake." Jesus' words were not a challenge; they were a statement of fact. You're either with him, or against him.
Not to decide is to decide. You cannot wage spiritual warfare in a half-hearted manner. The fence you want to sit on is topped with Satan's spikes.