the Congress of the United States, or Congress, the Legislature.
the Congressman, meaning a special one. Several congressmen and senators were there. In this sense the nouns are common.
Congressman illustrates De Vinne’s rule that a title not a synonym for a specified person (one only) should not begin with a capital. See [Association].
Centuries take a lower-case letter: fifteenth century, nineteenth, twentieth, etc. This is an exception to the rule concerning historical epochs, but custom has made it proper.
a club, meaning any.
the Club, meaning the Century Club.
Columbia College, Stanford University, etc.
the Constitution of the United States, of any particular state or society.
a constitution, meaning any.
the Continent, meaning of Europe, or when used as a substitute for the full name of any other continent.