We're gonna go out on a limb here and say that the first two sentences where President Kennedy talks about who he's proud to be standing around Berlin with shouldn't be considered the opening lines.

Sure, they're technically the opening lines and, yeah, they provide good context. But his speech really begins with sentence 3: "Two thousand years ago the proudest boast was civis Romanus sum."

This is apparently a thing that real, toga-wearing Romans used to say (as best we can tell, because none of them bothered to get it on video).

Basically civis Romanus sum meant literally, "I am a Roman citizen," in a really snotty way. As in, "How dare you treat me like this, I am a Roman citizen and therefore better than you!" Or, "I am a Roman citizen and thus deserve all the legal rights that Romans get." Since Rome was among the first places on earth with constitutional rights, this was the way people could demand that they be treated fairly…or at least by the law.

When Kennedy follows up his civis Romanus sum punch with his Ich bin ein Berliner jab, he's demanding better treatment for Berlin in a really old-school way. And he's saying that Berliners—all Berliners, not just those who happened to live on the Western side of the wall—should be able to say "Ich bin ein Berliner" and have it mean the same thing as "civis Romanus sum"—"I am a Berlin citizen and thus deserve all the legal rights that (both East and West) Berliners get."