The Law of the Gospel "fulfills," refines, surpasses, and leads the Old Law to its perfection. In the Beatitudes, the New Law fulfills the divine promises by elevating and orienting them toward the "kingdom of heaven." It is addressed to those open to accepting this new hope with faith - the poor, the humble, the afflicted, the pure of heart, those persecuted on account of Christ and so marks out the surprising ways of the Kingdom.
Source: Catechism of the Catholic Church (Vatican.va)