As Paul begins his second letter to the Thessalonian church, he acknowledges their afflictions and persecutions. But he also highlights the reason they are undergoing such troubles. It is for their benefit, so that they may be made worthy.
"This is evidence of the righteous judgment of God, and is intended to make you worthy of the kingdom of God, for which you are also suffering" (vs. 5). "To this end we always pray for you, asking that our God will make you worthy of his call..." (vs. 11)
But aren't the Thessalonians already made worthy through the sacrificial death of Christ upon the cross? Are they to now achieve what they have already received by their own righteousness and perseverance?
No, I believe this is a difference between justification and sanctification. The Thessalonians are saved already - but they must continue in this path and undergo the refining fire of trials to emerge on the other side as God desires them to be. In his grace, he has saved them already. Now is the time for that salvation to work in their hearts and manifest itself through a redeemed character and resolute faith. It is in this way that they will be made worthy.