Heide means "heath, moor." Was the der a Dative case, then? "Now comes the Savior from the boondocks -- er, wilderness?" Something like that? In the highly compressed language of poetry, especially to fit a meter, that could be it. It made sense, but seemed an odd thing for Bach to extol.
On the other hand, I knew that in English the words "heath" and "heathen" were closely related. A heathen was originally someone who lived rough, without a civilized home. Like marauding Danes rampaging through England. So, maybe it meant, "now comes the Savior from the nations." Of the nations? To the nations? In the German of Bach's time, were Gentiles sometimes called Heiden?
So, I looked it up today. Sunday's bulletin added a letter and dropped a comma. The song is Nun komm, der Heiden Heiland, Imperative verb and Genitive article = "Now come, the heathens' Savior." Okay, that makes more sense.
Well, I was close.
This is what grammatical gender can do to you. The difference between die Heide, "heath," and der Heide, "heathen" is the difference between feminine and masculine gender. Meanwhile, the definite article der can be 1) masculine singular nominative, 2) feminine singular genitive, 3) feminine singular dative, or 4) all genders plural genitive.
Personally, I dig the idea of a classy Christmas song about the Savior coming from the backcountry. But that, I guess, would be something like Nun komm, aus Heide Heiland.
This is why German occupies a place all its own on the Foreign Service Institute's learning difficulty index.