“But we must always thank God for you, brothers loved by the Lord, because from the beginning God has chosen you into salvation in sanctification of the Spirit and belief in the truth. Into this He also called you by our gospel—into obtaining the glory of our Lord Jesus Christ. So then, brothers, stand firm and hold fast to the traditions you were taught whether by our word or letter” (2 Thess 2:13-15).

In the first part of 2 Thessalonians 2, Paul delivered a grim condemnation of the godless who will fawn after the man of lawlessness—those who will be apostates from the heavenly to the earthly, the spiritual to the fleshly. Paul’s aim is not to trouble the Thessalonians with eschatological threats. His aim, in both his letters, is to reassure them. Following this north star, he takes another tack to stay on course.
Apostates from the spiritual to the fleshly come under a strong delusion. In contrast to these, Paul celebrates the fact that the Thessalonians were chosen into salvation from the beginning. Does Paul mean from the beginning of time or from the moment the Thessalonians heard the gospel? It isn’t completely clear. Context suggests Paul is saying from the time they heard the gospel. But when we believe, we come into Christ. Since Christ was chosen from the beginning, we share in God’s choosing of Him when we come into Him. We are treated as nothing other than sons chosen from the beginning in the Son.
Whatever beginning Paul has in mind, it is a point prior to any man of lawlessness, strong delusion, or apostasy. Whatever maelstrom may come, the Thessalonians have already anchored in the safe harbor of Christ. Paul wants them to know they are secure.
The Thessalonians were chosen “into salvation in sanctification of the Spirit and belief in the truth.” This is a dense and interesting phrase but may be simpler than it seems. If I jump into the water in the ocean it is clear that the water and ocean are one in the same. The water is the ocean is the water, etc. Similarly, salvation is sanctification of the Spirit and belief in the truth.
Truth is not a set of doctrinal propositions. Jesus is truth. Truth died and rose again. Paul tells us to consider ourselves dead to sin and alive to God in Christ (Rom. 6:11). As we believe in Truth Himself, the Spirit makes dying to sin and living to God a reality in us. This is sanctification or setting apart.
Salvation, then, is not just believing information. It is transformation. The living Christ within saves us from the operation of sin within.