What does it mean to be saved?
What does "elect" mean?
The Bible talks about many different salvations; some physical, of course, but the two important ones are spiritual salvation and the salvation of the soul.
Jesus died that all might be saved. Everyone. Not just a select few. Anyone can accept the offer. But, only many will.
But, his blood was shed for many. His blood was shed for the remission of sins.
If God chose only a few to be saved, then he also chose the others to remain unsaved. If I go to the store to get apples, the ones I don't select, I reject.
So, let's look at 2 Thessalonians 2:13: But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
Who are brethren? It's not all saved people, although many people refer to them as such. Matthew 12:50 tells us that it's those who are being obedient. Not all saved people are obedient.
"From the beginning". Firstfruit. Is everyone who is saved firstfruit? "God chose you as the firstfruits to be saved." You are quite correct that it's "you" that he "chose" in the verse, but he chose you to be firstfruits. (Paul talks quite a bit about firstfruits.) "First" fruits has to do with quality, not order, and if I remember correctly, it's a collective noun, just as "seed" is. When it's plural, it's more than one type of firstfruit (or seed), and when it's singular, it can be more than one, but only one type. So, "y'all (plural) are firstfruits (singular)" still makes sense, just as saying "y'all are corn seed". (Well, it makes sense grammatically, just not realistically.) If you were talking about individual kernels in a hanful of corn, it could be "corn seeds", but "corn seed", when referring to the hanful, while singular is still plural. (If you want more than the HoG so-so eplanation of "collective noun" in English, click
here.
"Salvation". What is salvation in this passage?
"Through sanctification." In "holying". Are we saved spiritually through being made holy? Or is being made holy come later?
"Sanctification of the spirit". Holiness of the sprit. It's not clear that this passage is talking about the Holy Spirit, but I believe it is. It's either "sanctification by the Spirit" or "sanctification of spirit".
"And belief of the truth". The word used here is "faith", and it's "faith in truth". There is no definite article.
How in the world anyone could apply this verse to spiritual salvation is beyond me, but many do. In spite of the fact that the book is written to the
Church at Thessalonica.