There was a remnant of Israel in Paul's day who were believers, and there always will be a remnant, there are always Jews who are believers even in our day. God is the one who kept/preserved this remnant, and the rest were given a state of blindness, and left to their own thinking. We must never be those who endorse, or even allude, to self righteousness as a part of salvation, or those who push one's work for God over God's work in and through them. If left on our own,in our own 'righteousness', we would be at enmity with God, we would not be for God, but against Him, and would be blind. God hardens by letting people be what they are, following their natural thinking and their natural bent against the truth.