Whom God hath set forth to be a propitiator through faith in His blood to declare His righteousness for the remission of sins that are past, through the forbearance of God.
The term propitiator means doing something for someone else; in our discussion text Jesus Christ is our propitiator through His shed blood for the remission of sin. Through Christ's shed blood He paid man's sin-debt, because man lacked the ability to pay his sin-debt.
Christ's blood is importance to our salvation because Christ's blood was pure and was the purifying agent that cleansed sin and He is God in the second person. His death was sacrificial as He was the lamb slain from the foundation of the world. God the Father did what He did out of love for humanity. Christ willingly wrapped Himself in humanity to pay man's sin-debt. What a loving God!
Christ's blood forgave sin whereas earthly priests sacrificed animal blood that lacked the ability to forgive sin. Christ and His blood met all of God the Father's requirements to atone sin; He was God and man as well as sinless. Christ's blood is the purifying agent cleansing man's soul from sin.
Given all that Christ done for humanity He is our propitiator who redeemed man from sin for that we owe it all to Him and to all I (we) freely give.
Christ atoning works made us free to serve God in true holiness and total devotion.
Christ our propitiator and Redeemer.
The word "intercessor" means someone who does something for someone else. Perhaps you are thinking of Romans 8:34 ("Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us"). The word "propitiator" specifically means someone who makes appeasement... that is, makes peace with or pacifies a belligerent party.