Once a Catholic always a Catholic - what does this mean and why do people say it?

Is Catholicism not a set of beliefs but more like a genetic condition? Is this a type of hypnotic suggestion to discourage people developing independent thinking and considering other faiths? Is the culture so deeply ingrained into children from such an early age that they are made to feel Catholicism is as much a part of their own identity as their name is? Just interested in your thoughts.