If we think that we can be righteous enough, by the way we live before God, we cut ourselves from the grace of God in Christ Jesus.
Of course, we have to aim to live righteously before God, but that should not be our focus, about our standing with God, because it is all about the righteousness of God in us through Christ Jesus, that God is looking at.
One of the men in Luke 18:9-14, thought that he can be righteous before God, and therefore accepted by God, just by the moral life he lived and the things he was doing for God. He thought ‘I can be righteous enough for God, so that God will accept me, because of the things I do for God’. So the grace of God, did not have place in his life, because he thought that he does not need it. The tax collector, was the poor in spirit man.
The other man, was repentant, and was looking for the mercy and the forgiveness of God for himself. He thought ‘Hey, I need the mercy and the grace of God for me, because I have done so many things wrong’. He just could not see himself able to be right with God, by the way he lived.
In Matthew 5:3, Jesus said that poor in spirit, have the Kingdom of heaven.
I believe to be poor in spirit, means that we do not think, that we can live righteously enough before God, ever, therefore, we need his grace in our lives. We can never be rich enough in our spiritual walk with God, so that we can fulfill the requirements of the Law, or keep God’s commands always, because it says in the Epistle of James, that if we brake one of the commands of God, we have broken all of them. And we know, that it is not possible for us, to keep all the commands of God, all the time. The moment we realise that, we become poor in spirit, and we need the grace of God, and his gift of righteousness in Christ, in order to be accepted by God as righteous.