When nouns don't require articles

Is there a general rule for when nouns don't require articles? For example, JFK famously said, "Ich bin ein Berliner", but that was incorrect. Another example, Lesson 10.6 "Sind Sie bereits Kunde unserer Bank?" I'm speculating that if one is talking about being a member of some grouping, e.g. Berliner, customer, soldier, etc. The article is not used. Please clarify. thanks, Byron

In lesson 2.4 it is explained that when someone is talking about their profession/title, it isn't preceded by an article. Nik uses the JFK "Ich bin ein Berliner" as an example. As far as any other cases, I'm not sure.

Yeah, I remember that lesson about professions. I'm wondering if that applies to any kind of grouping of people such as customer, client, etc.