Matthew 5:7 - Ellicott's Commentary On The Whole Bible

The merciful. — The thought is the same as that afterwards embodied in the Lord’s Prayer. They who are pitiful towards men their brethren are ipso facto the objects of the divine pity. The negative aspect of the same truth is presented in James 2:13. In this case, the promised blessing tends to perpetuate and strengthen the grace which is thus rewarded. No motive to mercy is so constraining as the feeling that we ourselves needed it and have found it.