Yes, which synod, please?
Seriously, I suspect you’re taking Mr. Lazareth a bit too literally. I would imagine that the point he was trying to make is that we must be careful about lumping all Christians and Christian things together when there are relevant distinctions to be made. And I think this is good policy, not just for Christianity, but for any grouping of people, things, ideas, etc.
However, sometimes, the distinctions aren’t relevant. I mean, for pete’s sake, if we can’t say all Christian things are alike in some ways, then how can we possibly talk sensibly about “liberal/conservative ideals”, “adult responsibilities”, or “human tendencies”? We can, and do, because within certain contexts, these very large groupings share common properties that can be discussed and considered without regard to their internal differences. Yes, often these terms do paint with too broad a brush, but in some cases, they can be perfectly acceptable.
For instance, we can say that Christian theology: is based on the idea that Jesus was the son of God, is monotheistic, uses the Bible as its primary sacred text (and believes the New Testament to be equally valid to the Old), shares roots with Jewish and Muslim theology, and so on.
But all that aside, it seems you’re objecting not to “Christianity” or “Christians”, even though they denote exactly the same broad ideological category as the adjective “Christian” does, because they’re nouns, and not adjectives.
This makes no sense, as pulykamell points out:
Indeed - why wouldn’t you be at a loss in either case? “Gee, he’s a Christian, but is he a Roman Catholic? An Evangelical? A Methodist?” How does the noun form impart more precision or detail than the adjective? The point is, it doesn’t.
So if you’re simply exhorting people to choose their words carefully, that’s fine. If you’re saying that words are meaningless if they’re general, that’s wrong. If you’re saying that general nouns are okay, but general adjectives aren’t, that’s just silly.