chapotraphouse

My understanding is they did that in response to Christians wearing crucifixes, so it was a conscious adoption of a religious/ethnic symbol in response to another one.

Yes that was exactly the case, a lot of what we know about old Germanic religion and culture might be either fabrication or late versions already being under the influence of christianity. The contacts with mediterranean cultures were much earlier, even the famous runes were derived from northern Italic script, most likely Etruscan, somewhere around I century BCE/CE.

Funnily enough the first symbol that was really used by all Germanic people is the christian cross, under that they unified into states, and that's also what is on every Scandinavian flag today.