Proxy–representative, substitute, stand-in, go-between.
Propitiation–our substitute who appeases the wrath of the offended person (God) and enables reconciliation (go-between)
whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed,
Therefore, in all things He had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people.
And He Himself is the propitiation for our sins, and not for ours only but also for the whole world.
In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins.