Matthew 5:7

Blessed are the merciful: for they shall obtain mercy--Beautiful is the connection between this and the preceding beatitude. The one has a natural tendency to beget the other. As for the words, they seem directly fetched from Psa 18:25, "With the merciful Thou wilt show Thyself merciful." Not that our mercifulness comes absolutely first. On the contrary, our Lord Himself expressly teaches us that God's method is to awaken in us compassion towards our fellow men by His own exercise of it, in so stupendous a way and measure, towards ourselves. In the parable of the unmerciful debtor, the servant to whom his lord forgave ten thousand talents was naturally expected to exercise the small measure of the same compassion required for forgiving his fellow servant's debt of a hundred pence; and it is only when, instead of this, he relentlessly imprisoned him till he should pay it up, that his lord's indignation was roused, and he who was designed for a vessel of mercy is treated as a vessel of wrath (Mat 18:23-35; and see Mat 5:23-24; Mat 6:15; Jam 2:13). "According to the view given in Scripture," says TRENCH most justly, "the Christian stands in a middle point, between a mercy received and a mercy yet needed.

— Jamieson-Fausset-Brown Commentary (public domain)