would probably be given for religious reasons in Germany, that is: denominational reasons. I can imagine quite a bit of mocking in the school yard, a boy called Luther
would be confronted with. For one reason it is practically non-existent as a first name, and in Germany we are not used to giving surnames as a forename, like you are in the States. Also, especially with parents who are very religious and emphasize their Lutheranism, little Luther
would probably be teased as pius (there are relatively few "pius" people in Germany, religion is not as present in society as in your country.) Of course this is no more than speculation as just about nobody bears this name.
is a completely different case, as fairly few people in Germany are aware of the fact, that this is another reformer's last name; they will much rather associate the name with Calvin
Klein and pronounce it as if it were English.)
The word Seibicke uses, is "bekannt," which in this case should rather be translated as "well-known" than "popular." Luther is not a German hero, not for most people.