J.I. Packer, from “The Heart of the Gospel” in Knowing God:
Has the word propitiation any place in your Christianity?
In the faith of the New Testament it is central.
The love of God [1 John 4:8-10], the taking of human form by the Son [Heb. 2:17], the meaning of the cross [Rom. 3:21-26], Christ’s heavenly intercession [1 John 2:1-2], the way of salvation—all are to be explained in terms of it, as the passages quoted show, and any explanation from which the thought of propitiation is missing will be incomplete, and indeed actually misleading, by New Testament standards.
In saying this, we swim against the stream of much modern teaching and condemn at a stroke the views of a great number of distinguished church leaders today, but we cannot help that. Paul wrote, “Even if we or an angel from heaven”—let alone a minister, a bishop, college lecturer, university professor, or noted author—”should preach a gospel other than the one we preached to you, let him be eternally condemned!” (“accursed” KJV and RSV; “outcast” NEB; “damned” Phillips—Gal. 1:8). And a gospel without propitiation at is heart is another gospel than that which Paul preached.
The implications of this must not be evaded.
Packer approvingly quotes John Murray’s explanation of propitiation:
The doctrine of propitiation is precisely this: that God loved the objects of His wrath (the world) so much that He gave His own Son to the end that He by His blood should make provision for the removal of His wrath. It was Christ’s so to deal with the wrath that the loved would no longer be the objects of wrath, and love would achieve its aim of making the children of wrath the children of God’s good pleasure.
—John Murray, The Atonement
Here is my layman’s definition for propitiation: the substitutionary death of Jesus turned the wrath of God into the favor of God. Or, “turn that frown upside down.”