I don't think it is necessarily bad grammer, maybe it is obsolete? Keep in mind I am not a native speaker.
"Risen" in this case is not a verb but acts more like an adjective, "the one who has risen". I always interpret sentences like this as "He is the one who has risen."
They're quoting the KJV of the Bible, it's incorrect in modern language, but I found this:
"It's an archaic use of English which conjugates verbs of motion with be in present perfect, in much the same way as French still does. The statement is the equivalent of "Christ has risen" and is stating a present-perfect fact."