2 Thessalonians 2:13-15

But we should always give thanks to God for you, brethren beloved by the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth. It was for this He called you through our gospel, that you may gain the glory of our Lord Jesus Christ. So then, brethren, stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us.