Do good, O LORD, unto those that be good, and to them that are upright in their hearts.
Do good, O Lord, unto (those that be) good. The confident trust expressed in Psalms 125:1-3 is the ground on which rests the prayer here. There is an inseparable connection between being "good" and receiving "good."
And to (them that are) upright in their hearts - not merely in outward conduct. The true "Israel" (Psalms 125:5), "such as are of a clean heart" (Psalms 73:1: cf. Psalms 7:10). The law was not mere letter, but spirit, even in the Old Testament; it is under the New Testament that the spirit of the law is brought fully to light. Thus the law commanded, Deuteronomy 6:5, "Thou, shalt love the Lord thy God with all thine heart," etc. And the tenth commandment concerned the inward man, "Thou shalt not covet," thereby detecting "lust" (Romans 7:7). The Psalmist repudiates the notion that mere descent from Abraham, and circumcision, can make any one of the elect nation accepted before God (cf. John Baptist's teaching, Matthew 3:9; and Romans 2:28-29).