Levite
Higher and Higher
I know that, but you have already stated your opinion on other groups, if you don't think other opinions are valid, that narrows the parameters of the discussion.
That's fine, but it is also just your opinion, we can't really say "Judaism believes"...in that context, only "these groups believe"...
I am honestly not certain which groups you mean here. If you mean Christian groups or "Jewish Christian groups," that is something that all the movements agree upon and the Tradition is fairly unanimous concerning: Christianity is another religion, and Jews are forbidden from practicing religions other than Judaism.
If you mean differing opinions amongst differing Jewish movements or communities, it's true it's often hard to say "Judaism teaches," in general, but it is occasionally possible, at least for the majority of normative Judaism. And not practicing other religions, or attempting to fuse those religions with Judaism, is pretty dead center of normative Judaism.