Mettle is the courage to carry on. If someone wants to "test your mettle," they want to see if you have the heart to follow through when the going gets tough.
Having the mettle to do something means you have guts. In short, you're a pretty impressive person. If you have the intellectual mettle to enter a political debate, not only do you know a lot about politics, but you have the spunk to show it off. Metal and mettle were used interchangeably meaning a solid material like gold, and the "stuff a person is made of" until everyone got confused and the words went their separate ways.