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.
Kingdom of God ...  This cannot possibly mean that the Thessalonians were not in the kingdom of God. Paul had specifically stated to them that they were "in Christ"  (1 Thessalonians 1:1), and no man was ever in Christ without being in the kingdom of God and of Christ.  Paul simply meant by this that their fidelity through sufferings would make and prove their worthiness of being in the kingdom.