The Lord appears to make two opposite claims in Mark 9 and Matthew 12. In Matthew 12:30, He says, "he that is not with me is against me." In Mark 9:39–40, He says, "he who is not against us is for us." We might describe the former as an exclusive statement, and the latter as an inclusive statement. It's worth thinking about these two statements.
It seems to me the difference between those two statements is the pronoun: when the Lord makes an exclusive statement, He uses the pronoun "me"; when He makes the inclusive statement, He uses the pronoun "us". In other words, when the issue is Christ Himself ("me"), then we can't be too exclusive. If you're not for Christ, then you are against Him, period. But when the issue is Christ and His followers ("us"), then we need to be inclusive. If you're not actively against them, then you are for them.
It seems to me there are two errors we might fall into here. The "liberal" error is to make an inclusive statement about Christ Himself, misquoting Matthew 12:30 as "he who is not against me is for me." The opposite error is to make the exclusive statement about a group, misquoting Mark 9:40 as, "he who is not with us is against us."
If we make an exclusive statement about a group, then we end up in some difficulty. We end up making loyalty to a group to be virtually the same as loyalty to Christ. And I know people who have had to deal with this: people who have been accused of defecting from the truth because they started to meet with a different group of Christians. Now, I may have problems with some of the fellowship decisions people around me have made, but to accuse someone of leaving Christ! I just don't see how someone who truly fears God wouldn't be terrified to say something like that.
On the other hand, if we make an inclusive statement about Christ Himself, then we're really denying the Gospel, aren't we? The Christian life centers on Christ. J. N. Darby wrote a paper called "Bethesdaism, or Indifference to Christ." I can't recall much about the article, but that title has haunted me for many years. What a description! How could a believer be indifferent to Christ?
Sadly, it happens. Sadly, it's a lot easier than we might think. I have found myself pretty close to indifferent to Christ many times, when I've let other things get in the way. It's all too easy for us to allow ourselves to find something that comes between us and the Lord.
But the point is, making an inclusive statement about Christ Himself (not about Christ and His followers) is really the first step to syncretism.
One of my daughters' friends was telling us that he was at some sort of interfaith event at a college, and there was a panel taking questions. He asked if each panel participant could briefly explain how their beliefs were divine, as opposed to being merely moral philosophies. In other words, is your faith really about God? Or is it just a moral code?
It seems to me that evangelicalism has reached a point where it's much more a moral code than a religion. When I was growing up, it was common to hear Christians claiming, "Christianity isn't a religion, it's a relationship." I'm afraid even "religion" would be an improvement for many at this point. It's remarkable how popular a Christ-optional Christianity is.
We're called to fellowship with the Father and with the Son (1 John 1:1–3). There is implicit morality there, but it's not really a moral code. Indeed, James 2:21–26 holds up Abraham (who was on the verge of killing his son as a sacrifice) and Rahab (who committed treason) as examples of faith. The moral code that comes from loving and fearing God might well be something the world around us finds incomprehensible, or even reprehensible.
But that's really the point: we're not called to a moral code, we're called to a Person.
So I'm trying hard to remember that when it's about Christ alone, I need to be exclusive. When it's about Christ and His followers, I need to be inclusive.