In Act III of All My Sons, what is shown by the fact that Jim has always known of Keller's guilt, but kept silent?
This is a highly timely and important revelation, as Jim's understanding of Keller's position and his own revelation about how he has been similarly involved in acts of compromise underscore the theme of moral compromise due to monetary greed. Keller is not alone in the world having done what he has done, as the character of Jim Bayliss makes perfectly clear. Note what he says to Kate in order to console her and to explain how he knows the truth about her husband:
We all come back, Kate. These private little revolutions always die. The compromise is always made. In a peculiar way. Frank is right--every man does have a star. The star of one's honesty. And you spend your life groping for it, but once it's out it never lights again. I don't think he went very far. He probably just wanted to be alone to watch his star go out.
Note how his own struggles with precisely the same issue make this central theme of moral compromise for money a universal issue. Compromise is accepted as a fact of life that cannot be ignored, but Jim's speech above makes clear the devastating consequences for those who do allow the light of their star of honesty to become extinguished.