2 Thessalonians 2:13-15

But we ought always to thank God for you, brothers and sisters loved by the Lord, because God chose you as firstfruits to be saved through the sanctifying work of the Spirit and through belief in the truth. He called you to this through our gospel, that you might share in the glory of our Lord Jesus Christ. So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter.