These verses come after, "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." (2 Thessalonians 2:13-14). The reason to stand firm is because they are already firmly secured by the love and gospel plan of the Father, Son and Holy Spirit. They could not be more secure.