The poor Jesus is referring to in Matthew 5:3 are not those who live in material poverty for there is no virtue in poverty itself. Furthermore, Jesus is not endorsing poverty as a superior way of life. The poor are not more favoured by God than others, though God constantly reminds us of our obligations to care for those who are poor.
The poor Jesus is referring to are poor "in spirit". They are impoverished of self. They are not full of themselves, but are empty of self. But why does Jesus use the word poor when he could have expressed his teaching in any number of other ways? When we see pictures of poor people in the Sudan or Ethiopia on the evening news we see people who are destitute, utterly dependent upon others, unable to alter their situation. They have no political or economic power to wield; they are not the movers and shakers in society. Their poverty has reduced them to beggars with outstretched hands pleading for help. If they are to live, someone with food must come to their rescue. It is the utter hopelessness of the poor that Jesus wants us to see, for their status perfectly reflects our standing before God. We are spiritually bankrupt, with nothing to offer God. We are without an acceptable standard of righteousness and are covered in our own sinfulness. Not a pleasant picture, but a true one nevertheless.
In the parable of the Pharisee and the tax collector Jesus portrays the Pharisee as "confident of [his] own righteousness." (Luke 18:9-14) The Pharisee considered his spiritual bank to be filled with deposits he had made. He thought his righteous life would be more than sufficient to secure him entrance into the kingdom of heaven, but Jesus declared him bankrupt. He has nothing going for himself. He can't save himself no matter how hard he tries. His attitude is the very opposite of that expressed by the hymn writer: "Nothing in my hand I bring/ Simply to the cross I cling/ Naked, come to thee for dress/ Helpless, look to thee for grace/ Foul, I to the fountain fly/ Wash me, Saviour, or I die." The poor in spirit are those who recognise their own sinfulness and exercise the only option they have: they cry out to God to save them. And in mercy, he does.
Theirs is the Kingdom of Heaven
The kingdom of heaven struck a responsive chord with Jesus' audience for it was a subject addressed by the prophets, and Israel waited in expectation of its arrival. However, the kingdom is not given to the powerful and influential or to those whose genealogy stretches back to Abraham. Possession belongs to those who recognise their spiritual inadequacy, those who are impoverished of self and therefore dependent upon the mercy of God. These are the people Jesus calls blessed or privileged, and to them is given the kingdom of heaven. Possession comes through submission.