Adam Clarke on Psalms 130:4

But there is forgiveness with thee - Thou canst forgive; mercy belongs to thee, as well as judgment. The doctrine here is the doctrine of St. John: "If any man sin, we have an Advocate with the Father, Jesus Christ the righteous; and he is the propitiation for our sins, and not for ours only, but also for the sins of the whole world." "Hear, O heavens, and give ear, O earth; for the Lord hath spoken!" Jesus has died for our sins; therefore God can be just, and yet the justifier of him who believeth in Jesus.