The first beatitude (Matthew 5:3). Greek ptōchoi tō pneumati — ptōchos denotes beggarly poverty (not the lesser penēs, "working poor"), and the dative tō pneumati localizes the poverty "in spirit" — i.e., in one's inner life before God. The phrase echoes Isaiah 66:2, "But this is the one to whom I will look: he who is humble and contrite in spirit and trembles at my word."
Poverty of spirit is not poverty in this world's goods; it is spiritual bankruptcy owned honestly. The poor in spirit are those who have stopped claiming assets before God. They know they have nothing to put on the table — no merit, no pedigree, no self-righteousness that passes inspection. The tax collector who wouldn't even lift his eyes but beat his breast and cried "God, be merciful to me, a sinner!" (Luke 18:13) is the textbook case. Jesus opens the Sermon on the Mount with this beatitude because every other beatitude and every other kingdom virtue is built on it. If you have not yet recognized your spiritual bankruptcy, you have not yet started. The blessing is surprising: "for theirs is the kingdom of heaven." Present tense. The kingdom is currently owned by beggars. Those who came to the throne with a bank statement were turned away (the rich young ruler); those who came empty-handed received everything. The gospel begins by making you poor so it can make you rich.