I want to deal with this argument in more detail:
Are you not aware that jews commonly referred to each other as "brother", whether they were beleivers or not? You cannot base Paul calling them "brothers" as meaning gentile believers. Research Pauls writing patterns.
While I did not argue that 'brethren' was an natural indicator of gentile believers, I find it interesting that you say to 'research Paul's writing patterns'. I agree. Lets do look at his writing patterns concerning 'brethern' in the rest of Thess.
1:9For they themselves shew of us what manner of entering in we had unto you, and how ye turned to God from idols to serve the living and true God
The 'you' and 'ye' are the brethren/elect of vs. 4. Thus we see the brethren/elect are those who 'turned from idols'. Could it be that Paul means this figuratively and that the 'jewish believers' turned from things which were the same as idols and now serve the living and true God where before they only thought they served the true and living God? Possible I guess. But the 'simple' or 'natural reading'? Clearly not.
The 'simple/natural' reading (ie. the reading which can be taken at face value without need for metaphors or figure of speech) would be that the brethren/elect are those who literally worshipped literal idols, and literally worshipped literally false and dead Gods. This would fit only gentile believers as a whole. In fact, it would tned to exclude jewish believers in the literal sense since literal idolatry was shunned by jewry of the time.
So, does it prove they are gentile believers? No. But if not, it certainly would force one out of the natural reading.
2:14For you, brethren, became imitators of the churches of God in Christ Jesus that are in Judea, for you also endured the same sufferings at the hands of your own countrymen, even as they did from the Jews
Notice the comparisons
1) The brethren are imitators of the jewish believer in Judea
2) The Jews persecuted the jewish believers in Judea
3) Likewise the brethren countrymen persecuted them
Now, if the brethren were jewish believers, then wouldn't "your countrymen" also be "the Jews"? Why then does Paul use language which makes it appear as if 'your countrymen' and 'the Jews' are two differenct groups? Again, the 'simple/natural' reading would that the brethren are gentile believers.
Also, the message of the rapture in I Thes 4 is to 'the brethren'. Who is the message of the rapture to in your opinion?
In short, the WHOLE book of I Thess is addressed to 'the brethren'. Unless you are going to argue that the audience of I Thess is jewish believers, then I can't see how you argue that its initial use is only to jewish believers.
Which brings me back to me question: IN CONTEXT, why do you say that the elect brethren of I Thess 1:4 is in reference to jewish believers only?