Tim Keller offers the following definition for what Jesus meant by being "poor in spirit":

It means seeing that you are deeply in debt before God, and you have no ability to even begin to redeem yourself. God's free generosity to you, at infinite cost to him, was the only thing that saved you.

But many people today resist Jesus' teaching about our spiritual poverty. Keller writes:

On the contrary, you believe that God owes you some things—he ought to answer your prayers and to bless you for the many good things you've done. Even though the Bible doesn't use the term, by inference we can say that you are "middle-class in spirit." You feel that you've earned a certain standing with God through your hard work. You also may believe that the success and the resources you have are primarily due to your own industry and energy.