The OT clearly teaches that God is a God of mercy (Dt. 4:31) and that he commands his people to act in the same way (Mi. 6:8; Zc. 7:9). Jesus said, ‘Blessed are the merciful, for they will be shown mercy’ (Mt. 5:7). He also said, ‘In the same way as you judge others, you will be judged, and with the measure you use, it will be measured to you’ (Mt. 7:2). Therefore in not being merciful to the poor they are heaping up strict judgment for themselves. They are not showing mercy in the worldly sphere; they will not receive mercy in the eternal sphere. That mercy triumphs over judgment is also the teaching of Jesus (Mt. 6:14–15; 18:21–35). By showing mercy to others now (which means exhibiting the character of God) they will discover that their own judgment has been reduced. Their cause is not hopeless nor is there any need for them to pile up their own judgment.
Carson, D. A., France, R. T., Motyer, J. A., & Wenham, G. J. (Eds.). (1994). New Bible commentary: 21st century edition (4th ed., p. 1360). Leicester, England; Downers Grove, IL: Inter-Varsity Press.