David had a perfect heart before the Lord and was always after God's mercy (1Kgs 11:4). He always sought God's mercy when he sinned (2Sam 12:22, Ps 109:21,26). David understood that mercy and goodness followed him all the days of his life (Ps 23:6) and we have those sure mercies of David (Acts 13:34). Mercy rejoiceth against judgment (Jm 2:13) because Christ was made the propitiation of our sins (Rom 3:25, 1Jn 2:2, 4:10). Propitiation means mercy-seat. Christ is our mercy-seat and it is His blood that was shed and sprinkled on the mercy-seat that satisfied the Father and made it possible for sinful man to receive God's mercy and grace for the forgiveness, complete cleansing and remission of ALL sin.
God does not deal with the believer according to his sin (Ps 103:10). He deals with the believer as with a son (Heb 12:7) according to what God has placed in the heart through grace (Heb 13:9) the moment of salvation (Eph 2:8,9). God will never judge the believer for his sin because His Son was already judged for all sin through death (Rom 8:3) and He has imputed His righteousness to the believer through faith (Rom 4:6,7). This is so difficult for so many to understand that God's goodness passes by all our concepts and understanding that we may have of Him through the natural mind.
We need to know the promises of God and to claim them as part of the faith that we have been given by the Son. Mercy has taken away what we deserve (wrath and condemnation) and grace has given to us what we don't deserve (God's favour). This is why, as a believer-priest, we must come boldly by faith to the throne of grace, obtain mercy, and find grace in the eyes of the Lord in the time of need (Heb 4:16, Rom 5:2). When sin abounds, God does not impute that sin unto us (2Cor 5:19, Rom 4:8) nor charge us with it, but makes grace much more (super-abundantly) abound toward us so that we can be free from that sin, not live in any condemnation associated with it and provide super-natural strength not to continue in that sin (Rom 5:15,20).
This is mercy rejoicing against judgment and it is the mercy and goodness that follows us and is sure and without partiality. God deals with us according to wisdom which is full of mercy and is justified of its children who are partakers of it (Jm 3:7, Lk 7:33-35).
Heb 9:11-17
11 But Christ being come an high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;
12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
13 For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:
14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
16 For where a testament is, there must also of necessity be the death of the testator.
17 For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth.