I shouldn’t actually have to post this theory since I mention it everytime we talk about Christian or Jacob, but here goes nothing.
I believe that Christian isn’t speaking for Jacob, I believe he took over from Jacob in a hostile way. The first time we saw Jacob he asked Locke to help him. The next time we saw him, Hurley stumbles upon the Cabin he looks inside and there he sees Christian smugly sitting in Jacob’s chair. Then a man’s eye appears in the window and scares Hurley away. The next time Locke goes to the Cabin he goes inside, alone, and finds Christian there instead of Jacob. Christian says that he’s speaking for Jacob, and he tells Locke what to do.
Christian has never given us a reason to trust him. I don’t think we should.