From Grace upon Grace:

Referencing the “poor” Matthew 5:3:

“The Greek word for poor is also the term for a beggar. [Note: beggars in the ancient world used to cry out 'Lord, have Mercy' which is why the 'Kyrie, eleison!' is an important part of the liturgy]. Those who are poor in spirit have no spiritual assets or credentials. They have nothing to offer to God the Father; the receive everything from Him. The poor in spirit are not spiritually rich and powerful; they receive the Holy spirit as beggars who ask for what they do not have. The Father’s kingdom is theirs as a gift, something that is always received and yet never possessed. unless they receive God’s kingdom, they can never enter it and reign in it as kings together with Christ (Mark 10:15; Luke 12:32; 22:28-30)”

“In our spiritual journey from the font to the grave, we walk before god the Father as beggars. Yet that is only half the story. We may be beggars, but paradoxically, we also associate with the holy angels. This wonderful paradox is confessed most clearly in the Divine Service by the curious juxtaposition of the Kyrie, ‘Lord have Mercy’, and the Gloria ‘Glory to God in the Highest..’ In the Kyrie and also in the prayers of the Gloria, we join the company of beggars who all appeal for mercy from Jesus as our advocate before the Father. In the Gloria we join with the angels who stand in adoration and joy before God the Father as they glorify Him.