You really can’t use the bible against christians

There are a few reasons for that. One, the big one to me, is inconsistency. You (collectively) claim this passage in Numbers is absolutely true, it means exactly what you say it means, and we have to defend our faith, which is impossible because you refuse to be proven wrong. But on the other hand when we point out passages that apply directly to you and show that you have to change your ways, all of a sudden it's a very old book with lots of contradictions that's been repeatedly copied with mistakes all over the place and can't possibly be reliable.

You can't have your kayak and heat it. Either it's a pile of old nonsense, in which case this Numbers passage is also a pile of old nonsense, or it's absolutely true in which case the stuff about Jesus being your God, and you have to repent, is also absolutely true.

Another is simply misunderstanding the text you're quoting. Numbers is part of the Pentateuch and doesn't apply directly to Christians. If you want to discuss the meaning of Numbers then you'll have to take it up with some religious Jews, because this is their text not ours. It's in the Bible for historical context so that we know something of Jesus' background. There is still some good stuff in the OT but it's called that - the OLD testament - for a reason, namely that it's been (sort of*) superseded by the NEW testament.

*not really, both testaments/covenants (~=contracts?) still stand, but why would you want to live under the covenant of law when you can live under the considerably better covenant of grace? It really makes no sense.