"A grace that is also judgment" -- Kittle

“For Paul ἱλαστήριον is not something which makes God gracious. This expiation for human sin presupposes the grace of God. For Paul even those who fall victim to the wrath of God are also set under His patience, kindness, and long suffering, Rom. 2:4. The ἱλαστήριον serves the revelation or the righteousness of God, cf. vv.25, 26: εἰς ἔνδειξιν, v.21: πεφανέρωται. But revelation and substitution are not antithetical. Revelation comes to men only as substitution is made. God in His righteousness reveals more than a patience which leaves sin unpunished, v.26. He also reveals a holiness which is at one and the same time both grace and judgment, which distinguishes between a sinner and his sin, which separates him from his sin, which brings him to a faith that is also repentance, i.e., self-judgment and true conversion. The revelation of a grace that is also judgment, and which establishes a faith that is also repentance, is no mere declaration of a transcendent attitude of God. It is a real fulfillment of grace and judgment on the human race. This demands, not only One to reveal God to the race, but also One to represent the race before God, to bear the divine judgment vicariously in order that the race might be brought thereby to self-judgment. A revelation without representation would be no more effective than the Law in terms of judgment. Hence it could not bring men true ἀπολύτρωσις. In this unity of the revelation of God to men and the representation of men before God, which really frees men from sin by self-release, redemption, and union with God, Jesus is ἱλαστήριον διὰ πίστεως ἐν τῷ αὐτοῦ αἵματι.”

Translation of the Greek:

ἱλαστήριον = “propitiation”
εἰς ἔνδειξιν = “in order to demonstrate/declare”
πεφανέρωται = “has been manifested”
ἀπολύτρωσις = “redemption”
ἱλαστήριον διὰ πίστεως ἐν τῷ αὐτοῦ αἵματι = “propitiation in His blood through faith”; Or, “a propitiation by His blood, to be received by faith”