Perhaps the two statements compliment each other to mean that there ARE NO NEUTRAL…
You are either for Christ, or against Him…whether you know it consciously or not.
You are either in a State of Grace, or you are not. You are either going to heaven, or you are going to hell. You are either Good or Evil.
You cannot be in between. If you claim to not be for Him, you BY DEFAULT, must mean you are against him.
And if you claim to not be against him, then BY DEFAULT, you must be for him.
There is no in between either way, is what I think it might mean. The statements are not contradictory, but rather complimentary: There is no neutral person. You are either for or against him, and if you are not one thing…you are by default the other.