Mr. Raymond who supposedly has liquor in his Coca-Cola bottle, really has just Coke. But he allows this rumor to be perpetuated because it gives the people a reason they can accept for his breaking the taboo of a white man living with an African-American woman:
Wh--on yes, you mean why do I pretend? Well, it's very simple...Some folks don't--like the way I live..It helps folks if they can latch onto a reason...When I come to town..if I weave a little and drink out of this sack, folks can say...[I'm]in the clutches of whiskey...He can't help himself, that's why he lives the way he does.
Mr. Raymond explains to the children that the townspeople cannot understand the real reason why he lives as he does: he wants to do this. Since doing so is socially unacceptable in the time of the setting in Alabama, he gives the people a reason they are willing to accept; therefore, he then has less conflict with them.
Virulent racism is so endemic to the culture of the American "Deep South" during the period depicted in Harper Lee's To Kill a Mockingbird that any suggestion that a white citizen would prefer the company of African Americans would be seen as traitorous, deranged, or both. Such is the case with the character of Mr. Dolphus Raymond. Raymond is the town drunk, regularly seen with a brown paper bag containing a bottle filled with whiskey, which he would consume through two yellow straws protruding from the bag. His reputation as the town drunk excuses, in the eyes of Maycomb's white population, Raymond's tendency to hang out with the African American community, which would otherwise be scandalous because he is white. Some of the town's white population explain Raymond's preference for alcohol and the company of blacks as a reaction to his fiancé's suicide on the eve of their wedding.
In Chapter 20 of Lee's novel, Mr. Raymond reveals the secret he has kept for years. Overhearing Dill complain about his stomach, the town "drunk" offers the young boy a drink from his bottle. Dill is surprised, to say the least, to discover that the bottle contains Coca-Cola rather than the whiskey everyone assumed was the contents of Raymond's ubiquitous bottle. Responding to Scout's question about the reason for the ruse, Raymond explains,
Some folks don't like the way I live. . . I try to give them a reason, you see. . . When I come into town, which is seldom, if I weave a little and drink out of this sack, folks can say Dolphus Raymond's in the clutches of whiskey -- that's why he won't change his ways.
In short, then, Raymond uses his ruse to provide an excuse for preferring to remain in the company of the town's African American community. So morally corrupt is the town's white population, with the notable exception of Atticus Finch, Scout's father, that a decent white man has to pretend to be drunk in order to prefer the company of blacks.