Paul wrote the letter to correct their doctrine.
Rom 1:
7) To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ.
1 Cor 1:
2) Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours:
2 Cor 1:
1) Paul, an apostle of Jesus Christ by the will of God, and Timothy our brother, unto the church of God which is at Corinth, with all the saints which are in all Achaia:
Gal 1:
2) And all the brethren which are with me, unto the churches of Galatia:
Eph 1:
1) Paul, an apostle of Jesus Christ by the will of God, to the saints which are at Ephesus, and to the faithful in Christ Jesus:
Phil 1:
1) Paul and Timotheus, the servants of Jesus Christ, to all the saints in Christ Jesus which are at Philippi, with the bishops and deacons:
Col 1:
2) To the saints and faithful brethren in Christ which are at Colosse: Grace be unto you, and peace, from God our Father and the Lord Jesus Christ.
1 Thes 1:
1) Paul, and Silvanus, and Timotheus, unto the church of the Thessalonians which is in God the Father and in the Lord Jesus Christ: Grace be unto you, and peace, from God our Father, and the Lord Jesus Christ.
2 Thes 1:
1) Paul, and Silvanus, and Timotheus, unto the church of the Thessalonians in God our Father and the Lord Jesus Christ:
The seven church epistles (Romans through Thessalonians) were written to Christians (saved believers).