In the beatitudes, Jesus describes the supremely happy life enjoyed by all who embrace Him as king and enter His kingdom by grace. The first characteristic of those who live the truly good life: blessed are the poor in spirit.

Resources:

Matthew 5:1-3

Studies in the Sermon on the Mount by Martin Lloyd-Jones
The Sermon on the Mount by Sinclair Ferguson
The Gospel According to Matthew by RT France
A Commentary on the Psalms by Alan P. Ross