Paul then illustrates the fact that not the whole of Israel has been cast off by reference to 1 Kings 19:10; 1 Kings 19:14; 1 Kings 19:18. It was in the portion read in the synagogues under the heading ‘Elijah' (compare Mark 2:26, ‘epi Abiathar'). There the Scripture states that when Elijah had thought that he was left on his own as the only one who was faithful to God, God had replied that ‘I have left for myself seven thousand men, who have not bowed the knee to Baal.' These were the faithful, the elect remnant who constituted the true Israel with whom God was ready to deal. It is significant that, as Paul was well aware, a hundred or so years later the nation as a whole would be swept away in a number of captivities, with large numbers soon no longer owing allegiance to YHWH. And we must remember that they had been swept away because of their idolatry which demonstrated that they had turned their backs on YHWH. Only the upright in heart would take steps to continue their allegiance to YHWH in the conditions which ensued. Note that Paul's citation is an abbreviation of the relevant verses in LXX.
‘Baal' has the feminine article. This was a practise among the Jews. The purpose of it was in order to warn a reader not to pronounce the name but to substitute it, possibly by ‘bosheth' (thing of shame). At one stage using the name of Baal was considered a thing of shame. As it is doubtful if Paul followed the practise it must have been in the copy of the LXX that he was utilising.