Probably the first four beatitudes do not refer to four distinct groups of people: the poor (in spirit), the mourners, the meek and those who hunger and thirst. These are perhaps four examples of a large group of the poor and the oppressed people. The first (and the second) beatitudes reflects Isaiah 61: 1 which speaks of the prophet’s mission to the poor.
In the first beatitude Jesus proclaims the blessedness of the poor in spirit. By adding “in spirit” Matthew has not substantially changed the meaning of the original beatitude (Cf. Lk 6: 20). He has highlighted the spiritual and religious implications of the word poor (Cf. Ps 69: 32-33). “The poor in spirit,” for Matthew, are the pious and the devout people who are also materially poor. They know their need for God who is their protector and vindicator, and therefore they seek God and place their trust in him. Jesus brings Good News to such people (Cf. Mt 11: 5 // Lk 7: 22; Lk 4: 18) and here in Matthew 5: 3 he pronounces a blessing upon them. The expression “poor in spirit,” then, does not refer to those who have the spirit of poverty or to those who are perhaps detached from their wealth. Voluntary poverty and detachment from wealth have value in the light of other Gospel texts (eg. Mt 19: 16-29). What the Matthean expression refers to is something different. “The poor in spirit” is to be understood primarily in relation to God. It expresses an attitude of dependence upon God in the context of poverty.
To the poor in spirit Jesus promises the Kingdom of Heaven. In fact he assures them that the kingdom with all its future blessings is already their possession. It is as if the end-time blessings of the kingdom are already realized in the case of the pious and the just, namely, the poor in the spirit. More than being a statement of fact, the beatitude in Matthew 5: 3 is an exhortation and invitation to all Jesus’ followers to live according to the norms of piety. Probably it is for this reason that Matthew has emphasized the religious implications of the word poor in the beatitude.