In 1942 Karl Barth was asked that question in a letter from an American Pastor. In October Barth replied in a lengthy note (which also answered other questions the unknown Pastor asked. Barth simply addressed the letter to a ‘dear friend’.)
It is an interesting question and Barth’s answer runs to 3 big pages.
This letter, and many other brilliant and timely pieces are included in a volume titled Eine Schweizer Stimme. The book contains little occasional pieces by Barth during the war years, 1938-1945.
Barth may have been wrong about some things and he may have been too dismissive of his theological superior, Brunner. And he was certainly wrong to disdain Zwingli. But one thing you have to give the guy- he was engaged with what was going on in the world and he applied theology to life. Unlike so many of our contemporaries in the fields of theology and biblical studies.