Matthew 5:3-12 lists eight Beatitudes that were spoken of by Jesus. Jesus said, “Blessed are the poor in spirit: for theirs is the kingdom of heaven.” (Matthew 5:3)

To be blessed, one must have an attitude to be poor in spirit. In other words, one must understand that their happiness comes for their acknowledgment of their own spiritual need; that they are poor without Christ Jesus and that only in Christ Jesus is the kingdom of grace and the kingdom of glory theirs.
It is the acknowledgment of one’s spiritual poverty i.e., the need for Jesus, and the confession of the same, that leads to inheriting the the kingdom of heaven (of grace and glory).

We must always have a need for Jesus i.e., we must always be poor in spirit, so that the kingdom of heaven (grace and glory) can be ours.

Matthew 5:3 (KJV)
3 Blessed are the poor in spirit: for theirs is the kingdom of heaven.