13 But we are bound to always give thanks to God for you, brothers loved by the Lord, because God chose you from the beginning for salvation through sanctification of the Spirit and belief in the truth WEB
Paul knew that the Christians in Thessalonica were genuine believers who had the Holy Spirit living in them because of their daily transformation by the Spirit of God through the process of sanctification which gave evidence of their salvation.
The word sanctification means to be set aside for God’s use. It also means to be holy. Sanctification has two processes.
1.) A person becomes holy the moment they place their faith in Jesus Christ. The Holy Spirit takes residence in their heart (the inner man), making their spirit holy.
2.) Sanctification in the flesh is a process of a daily walk with the Lord in obedience to the Scriptures which molds the believer into the image of Christ Jesus. If the person makes a daily habit of communicating with the Lord through prayer and reads the Word of God to hear from the Lord and walks in God’s commands, the process of sanctification will continue for the flesh. If not, it will hinder their spiritual growth.
Paul and his companions always prayed to the Lord with thanksgiving for the brothers in Thessalonica for their faithfulness to the truth.