"This is the downfall of your interpretation of this verse: Since the clauses are not adverbial (which would answer "how") and since these clauses are, in fact, dative clauses of the dependent type, you have no standing to try to determine "how." The text of this verse doesn't address the question."
The Archangel
There are no adverbs in the passage? en + dative indicating manner is adverbial thus "through (en) sanctification (dative) describes the manner of God's choice, and [through (en)] faith (dative) describes the manner of God's choice.
ἐν + the dative does not always indicate manner and it is not always adverbial. Here's why:
ἐν + the dative is adverbial in a so-called "dative of manner." This cannot be a dative of manner because the dative clauses cannot be turned into an adverb in English--we could not say God chose...sanctificationally or beliefially. So, one test of the dative of manner does not pan out.
Secondly, an adverbial phrase does answer the question "how." But these dative clauses cannot answer that question because of what they modify. The immediate antecedent is not the verb "chose" but is the accusative noun "salvation." Since these dative clauses modify a noun, and not a verb, they cannot be adverbial
The phrases (ἐν + the dative) are prepositional phrases first and foremost and, as such, modifying "salvation" they describe the means of salvation, not the means of God's choosing. (See further discussion below)
Further, this must be a dative of means, not a dative of manner (adverbial) because the main verb (chose) is middle/passive. A dative of manner does not, to my knowledge, occur without an active verb. The use of the dative of means with the middle/passive verb indicates agency on the part of the subject--God, in this case. God is the Agent choosing persons to save and then accomplishing that salvation by sanctifying them and causing them to believe in the truth.
What is more, the preposition for "through" is δία + the genitive. We have neither δία nor the genitive here, therefore your assumption that "through" can be assumed is incorrect.
So, your grammatical assumptions fall far short of any grammatical reality.
The verb is Aorist, indicating the Thessalonians in view had been chosen, but does not rule out that God continues to choose people in the same manner, just not as first fruits.
It is clear you don't understand the Aorist. The Aorist indicates a snapshot of past time, a completed action. The Aorist use of the verb "chose" completely and totally rules out the concept of a "continual choosing."
Either reading -as first fruits or from the beginning - does not alter my understanding, but as first fruits more clearly puts the timing of the choosing as during their lifetime, I like that version best.

as to the clause from the beginning not providing the time of the action, and therefore not adverbial, seems like a moot point.
"As first fruits," if that is the best rendering, cannot put the time of choosing within the lifetime of the ones being saved. The Aorist verb will not allow for that. The nature of the Aorist verb is partly why I believe "from the beginning" is the better rendering. Regardless, the rendering the accusative noun cannot alter the Aorist-ness of the verb.
Yes, you can agree with me that we were chosen for salvation, that is how the NASB puts it. That gives us the purpose of God's choice.
"Chosen to be saved" is the clear reading of the text. And, as you will note, I am not agreeing with you; I am agreeing with the text. If you happen to agree with the text too, then we might be said to be in agreement on this one point.
I will not dispute whether the datives are datives of manner or of means, both relate to how God chose and thus are adverbial.
Wrong. As I mentioned before, these dative phrases are prepositional phrases and are modifying the noun "salvation." Therefore, since they are not modifying a verb, they cannot be adverbial. In any event the dative of means, can (and, I think, does) apply to the prepositional phrases precisely because the phrases give the means to how one is saved--sanctification and belief in the truth, not the means of how one is chosen.
To add a further clarification. Generally, prepositional phrases are adjectival (as opposed to adverbial) when they expand the object of the preposition with further nouns--which is exactly what happens here. These phrases (which are really one phrase since ἐν governs both the dative nouns) are not mere ἐν + the dative. Rather, these phrases clarify the noun "salvation."
We are not "sanctifying" ourselves. We are being sanctified by the Spirit. We are not required just to "believe." We are required to believe in the truth. Both these phrases expand on the meaning of the noun "salvation" and show how the salvation is accomplished. Also, the word order of the sentence itself gives credence to this.
What is more, in v. 14 we see the phrase "to this he called you..." The "this" is an accusative relative pronoun and, being neuter, refers to the entirety of "salvation through sanctification...and belief..." This is important because we see "God calling" and God is not calling them to chose them. Rather, He is calling them to salvation.
In short, the "how" we were chosen is not because of our sanctification or belief (especially because God is the Subject-Actor here).
On the contrary, we are set aside in Christ by the Holy Spirit baptizing us into Christ. Thus the adverbial clause does tell us how God chose us - by putting us in Christ and by basing His choice on crediting our faith in the truth as righteousness. QED[/QUOTE]
There is no adverbial clause here. The only "how" question is not how we were chosen, but how we are saved. The "set aside in Christ..." is not addressed in this passage and is therefore under the category of facts not in evidence. Whether I agree that we are set aside in Christ or not (which I do, but not in the time-frame you suggest) is moot.
The Archangel