For most Christians, their faith is primarily about what Jesus has done for them. Jesus died on the cross to pay for their sins as an act of grace so that they could be restored to a right relationship with God and have eternal life. This is not something you can earn; it is a free gift that you can accept by putting your faith in Christ. So it seems somewhat strange that Paul talks about being “worthy of your calling” in the first chapter of his second letter to the Thessalonians:
We boast about you in the churches of God because of your endurance and faith in all your persecutions and in the hardships which you suffer. This is a clear sign that God has judged correctly that you will be counted worthy of the Kingdom of God, for which you are suffering… This is why we constantly pray for you, asking God to make you worthy of your calling, and to give you power to do all the good things that you want to do and every act prompted by faith.
Paul is clearly talking to people who have already put their faith in Jesus, so he is not telling them to do something in order to “earn” the salvation which they have already received as a free gift. He is, however, dealing with the reality that most people do not understand or agree with the Christian message, and as a result those who do put their faith in Christ often suffer misunderstandings, hardships and persecutions. Those who endure are those who will be counted worthy.