More from Moo

... so as to be just and the one who justifies those who have faith in Jesus (Rom. 3:27). 

Moo explains

Paul's point is that God can maintain his righteous character even while he acts to justify sinful people because Christ, in his propitiatory sacrifice, provides full satisfaction of the demands of God's impartial, invariable justice.  To be sure, this way of viewing the atonement is out of fashion these days, frequently being dismissed as involving ideas completely foreign to the biblical teaching about God's sovereignty and love.  But whatever the mistakes of Anselm of Canterbury, whose famous treatise Cur Deus Homo ("Why God Became Man") is widely regarded as the fountainhead of this approach, his emphasis on the divine character as incapable of dismissing sin lightly is a vital component in the biblical doctrine of God.  Those who ignore or minimize the problem inherent in a holy God accepting sinners may well heed Anselm's own warning: "You have not yet considered the weight of sin."