This is an obvious sign of the righteous judgment of God, to the end that you may be counted worthy of the Kingdom of God, for which you also suffer.
Thayer says the word "righteous" (dikaios) means "rendering to each his due...passing just judgment on others" (149-1-1342) and "judgment" (krisis) refers to "judgment that is opinion or decision giving concerning anything" (361-2-2920. Paul is saying, "Your faithfulness under severe persecution and testing is a token of God’s righteous judgment, given with the purpose that you may be counted worthy of His kingdom." The very fact that these believers stand true and unshakable under such terrible persecution assures Paul they are worthy of the kingdom of God.