At one point in the Gospels, the disciples come and tell Jesus that they found a man who was casting out demons and doing all kinds of amazing things in His name, but that they stopped him because he was not one of the disciples. That is, he wasn't in the inner circle. And Jesus told them, "Don't stop him. Whoever is not against us is for us." (Mark 9, Luke 9)
This would seem to say that Christ was only concerned about those who were vocally or actively against what He was doing, that anyone who was not staunchly opposed to His work was no problem for it.
But then, at another point in the Gospels, the Pharisees begin talking about how Jesus must get His power from the Devil himself, casting out demons by the power of the head demon. When Jesus responds to them, He says, in part, "Whoever is not with me is against me." (Matthew 12)
And all of a sudden...what?
If you are not against Him, you are for Him, but if you are not with Him, you are against Him. That gets a little thick to wade through.
What we cannot say to ease the tension between these two statements is that they were spoken in two very different contexts and so, each is true according only to the situation in which Jesus spoke it. We cannot say this because it introduces both a circumstantial element to God's nature and a contradictory element to His character, neither of which can be true if God is, in fact, God. That is, the God who is the same yesterday, today, and forever cannot be changed by circumstances and cannot contradict Himself.
Yet we have to say something, don't we?
These statements, when taken in their respective contexts, are actually increasing depths of the same statement, not contradictory ideas. They build on one another and draw us closer to the Christ, but only if we see what was actually going on here.
In the first statement, a man is using Jesus's name to cast out demons, even though he is not part of the formal discipleship. In other words, the disciples don't recognize him. Who even is this guy? But Jesus says not to stop him because whoever is not against Him is for Him.
What's really happening here is that Jesus recognizes that the man is using the name of Christ to do a rather Christ-ly work. He is not misusing the name of Jesus, and so every demon that he casts out is a call for the people to come to know more about the name by which the demons were cast out. What this man is doing is creating a pathway to Christ, not one that leads away from Him, and this can only be a good thing. This can only be done for Him, for it leads to and not away. Thus, even though the man is not one of the disciples, he is not against them and he is not working against them. He is actually working for them, even if he doesn't know it. Persons will come to Christ because of this man.
And yet, there is another very real truth at play, as well, and that is reflected in the second statement of Jesus - whoever is not with me is against me. Anything that does not come from an intimate fellowship/relationship with Christ is something dramatically lesser. Only what comes directly from Him is the fullness of itself; everything else is less.
So if we take this back into the context of the man who was casting out demons, Jesus recognizes that although this man's actions may help to pave the road toward Christ, not everyone will walk it. Some will be satisfied with just hearing the name, with just having the demons cast out, with witnessing the man and not the Man of God. And in this sense, the man who is for Him because he is not against Him is also against Him because he is not with Him.
Put it into the context of the Pharisees, who are the real target of Jesus's words (rather than the devil).
To their own minds, they are doing nothing wrong by not being Jesus believers. They are trying to keep the religion pure, trying to keep the people on track, trying to keep the Jews faithful. They are doing all of the "right" things, and in their minds, this makes them friends of the Kingdom of God, not enemies of it. But they are not "with" Jesus. Not by a long stretch. And Jesus here says plainly, "If you are not with me, you are against me." In other words, if you are not on board with the Messiah, you are against the Kingdom of God, the very thing the Pharisees believed they were most "for." This is the second statement.
But the truth goes even deeper, back to the first statement. They are clearly against Him. They are blazing a path away from Him and making it really easy for others to follow them down that road. At every turn, they question and refute what He tries to teach. There is no universe in which you could say they are for Him, for they are clearly against Him.
These two passages, though they seem contradictory to us, are nothing of the sort. They are complementary. They build on one another, no matter which side of them you fall on. Sometimes, we are not against Jesus and may even be cutting a path toward Him, but if the world is content to stop short and never truly encounter Him, we are fundamentally against what He is doing. If we never actually take the world down that road, they never meet Him, and we have done detriment to His work. Though we were for Him, we were against Him.
Or maybe we think we've got it right and don't need Jesus. Maybe we aren't Christian, but we still have an interest in "holy" things. We claim that you have God without the Christ or even that you can have the Christ without the Cross, and in this, we are not with Him. It doesn't take long from here before we are blazing a path away from Jesus and inviting others to follow. Because we are not with Him, we cannot be for Him. We are against Him, through and through.