Part of what the Lord Jesus is doing in the Sermon on the Mount is “describing the Christian character that flows from within.” (Warren Wiersbe, The Bible Exposition Commentary, page 21)
Let’s take these verses one at a time. When the crowd heard Jesus say “Blessed” they were hearing something that described “divine joy and perfect happiness.” (W. Wiersbe)
“Blessed implies that kind of joy that is described as inner satisfaction and sufficiency that did not depend on outward circumstances for happiness. This is what the Lord offers those who trust Him!” (Warren Wiersbe) These attitudes that Jesus describes here, are those we need in our lives.
3 “Blessed are the poor in spirit,
for theirs is the kingdom of heaven.
An attitude of being “poor in spirit” really means to be humble or to have the correct attitude of oneself. What is that attitude? In part, Romans 12:3 says, “...Do not think of yourself more highly than you ought…” This phrase “poor in spirit” doesn’t mean what the world thinks it means, but it is being honest with ourselves, recognize that we are who we are and that we are not someone else or what others think we are. We are hearing Jesus say, we are not self-sufficient but we should have the characteristic of deep humility of seeing ourselves as someone who is utterly bankrupt apart from God. It reminds us of our great sinfulness, lostness and hopelessness without Christ and His righteousness He freely gives. Our first attitude should be to see ourselves as empty of purpose and worth without Christ in our lives.
Once having come to that understanding then God says that we will be given the “Kingdom of Heaven.” What does that mean? Jesus is teaching right up front that the Kingdom of Heaven is, which He has preached before, (Matthew 3:2, “and saying, “Repent, for the kingdom of heaven has come near.”). Jesus says for those who are willing to admit their sin and repent, God’s love and forgiveness brings you into that “kingdom” that God reigns and rules over, the lives whose hearts have humble themselves before God.