And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.
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Theophylact of Ochrid
The Cross will then be seen in heaven shining more brightly than the sun as a reproof to the Jews, for when the Lord comes He will display the Cross as the strongest evidence against the Jews, like one who shows the stone with which he was struck. He calls the Cross a sign, as a trophy and royal ensign. Then all the tribes of the land of Judea will mourn and bewail their own unbelief, and all those who care for earthly things will mourn, though they be Christians. For those who are attached to earthly things might also be called "the tribes of the earth." Although the Lord comes accompanied by the Cross, He also comes "with power and great glory."