John Chrysostom

But because the sun will be darkened, the cross would not be seen, if it were not far brighter than the rays of the sun. That the disciples might not be ashamed, and grieve over the cross, He speaks of it as a sign, with a kind of distinction. The sign of the cross will appear to overthrow the shamelessness of the Jews, when Christ shall appear in the judgment, shewing not only His wounds, but His most ignominious death, And then all the tribes of the earth shall mourn. For when they shall see the cross, they shall bethink them how they have gained nought by His death, and that they have crucified Him whom they ought to have worshipped.