That's an interesting question in it's own right, but is different in this sense: Friga, Woden, etc are all deities. Christians believe in a deity. On the other hand, an atheist referring to Woden or Friga is a different class of phenomenon.

Returning to the government official, if the President says, "Good-bye" is that different from the president saying, "Good be with you"? If the President were an atheist and said, "God be with you" then he is either being a complete hypocrite and ass, or he is just mouthing a common platitude to fill airspace. If the President is a devout theist of some sort and says, "God Be with you" then he is invoking his deity's power to influence your life. That would be a religious expression.

Which one is the greater threat to the 'separation of church and state'?