In order to have been an adage, it must have been old. 'There is no such thing as a young adage,' (Sir Walter Scott, The Fortunes of Nigel). In any event, Churchill did not use the definite article or the first-person plural. He said, "'Let us therefore brace ourselves to our duties, and so bear ourselves that if the British Empire and its Commonwealth last for a thousand years, men will still say, 'This was their finest hour.' "
Let us brace ourselves to our possessives. Robert Paul
----- In Latin, Churchill's adage is false.