God's righteousness binds together his love and his wrath.

We have seen that God's love always observes the boundaries of his righteousness. Even in redemption, god takes enormous pains so that in showing love he may be just (Romans 3:26). the sacrifice of Christ ensures that God's redemption is both loving and righteous, so that Scripture can even appeal to God's righteousness as a ground for the forgiveness of sins, and thus God's righteousness becomes a form of love. But it is also God's righteousness that ensures  the final punishment of those who reject his love, that is, his wrath against them. Without the wrath of God against those who finally disbelieve, God's love is no longer righteous. So God's righteousness binds together his love and his wrath. God's righteous love must be wrathful, if at the end of history there remain any unrepentant people. 

John Frame, Systematic Theology pg. 275