The text sets up a scenario that plays on the common social expectation to say 'Bless you' after someone sneezes. It creates a humorous situation by implying uncertainty about the proper etiquette when God sneezes, as 'Bless you' is typically said to a person who sneezes, and it would be ironic to say it to God.
You go to heaven . . . God sneezes . . . What do you say ?