Romans 9, 10 and 11...
Rom 11:7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded
Rom 11:8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day.
Rom 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
The elect of Israel followed Christ and those who did not were, of course, not the elect.
Rom 11:8-10 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day. And David saith, Let their table be made a snare, and a trap, and a stumblingblock, and a recompence unto them: Let their eyes be darkened, that they may not see, and bow down their back alway.
This appears to be punishment for rejecting the Messiah.
Rom 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
Indeed, it would be conceit and "that blindness in part is happened to Israel" for someone to think he is elect just for a being a Christian.