The teachings of Jesus in the Gospel of Saint Matthew begin with the promulgation of the Kingdom of God in the Sermon on the Mount, this invitation to enter into the Kingdom of God is expressed in the famous 8 beatitudes or promises. The Kingdom of God begins with this teaching: "Blessed are the poor in spirit, for theirs is the kingdom of heaven", Matthew 5:3.
Thomas Aquinas teach us about the beatitudes: "all the beatitudes belong to the perfection of the spiritual life", and this corresponds to poverty, to leave everything for the Kingdom of God.
There is a teaching of Jesus about poverty, the story of the young rich:"Just then a man came up to Jesus and asked, "Teacher, what good things must I do to get eternal life?". "Why do you ask me about what is good?" Jesus replied. "There is only one who is good. If you want to enter life, keep the commandments". "Which ones? he inquired". Jesus replied, "You shall not murder, you shall not commit adultery, you shall not steal, you shall not give false testimony, honor your father and mother, and love your neighbor as yourself". "All these I have kept", the young man said. "What do I steal lack?". Jesus answered, "If you want to be perfect, go, sell your possesions and give to the poor, and you will have treasure in heaven. Then come, follow me". When the young man heard these, he went away sad, because he had great wealth". Matthew 19:16-22.
Jesus calls us to avoid the pride that comes from the accumulation of wealth: "Someone in the crowd said to him, "Teacher tell my brother to divide the inheritance with me". Jesus replied, "Man, who appointed me a judge or an arbiter between you? Then he said to them, "Watch out" Be on your guard against all kind of greed; life does not consist in an abundance of possessions", Luke 12:13-15.
We must understand the concept of poverty of spirit as "spiritual begging", as an attitude of humility and devotion to the wisdom of God. To have poverty of spirit corresponds to the gift Piety or Fear of the Lord, as Thomas Aquinas explains in the Summa Theologiae. This is why it is the first beatitude. The Kingdom of God begins with the Piety: "The fear of the Lord is the beginning of wisdom" Proverbs 9:10.