1:
Phrase: B.C., A. D.
Reason: B.C. is "Before Christ" abrreviated, and A.D. stands for "Anno Domini" which is Latin for "year of the Lord"

2:
Phrase: Merry Christmas!
Reason: DUH

3:
Phrase: Gesundheit! or Bless you!
Reason: Saying Gesundheit to some one arises from an old belief that when a person sneezes, they are trying to get rid of a devil inside of them. If someone doesn't believe in God, it's well nigh impossible to believe in the devil.