What is a Propitiation?

1 John 2:2

And he is the propitiation - ‘Ἱλασμος· The atoning sacrifice for our sins. This is the proper sense of the word as used in the Septuagint, where it often occurs; and is the translation of אשם asham, an oblation for sin, Amo_8:14. חטאת chattath, a sacrifice for sin, Eze_44:27. כפור kippur, an atonement, Num_5:8. See the note on Rom_3:25, and particularly the note on Luk_18:13. The word is used only here and in 1Jo_4:10.

And not for ours only - It is not for us apostles that he has died, nor exclusively for the Jewish people, but περι ὁλου του κοσμου, for the whole world, Gentiles as well as Jews, all the descendants of Adam. The apostle does not say that he died for any select part of the inhabitants of the earth, or for some out of every nation, tribe, or kindred; but for All Mankind; and the attempt to limit this is a violent outrage against God and his word.

For the meaning of the word παρακλητος, which we here translate advocate, see the note on Joh_14:16.

From these verses we learn that a poor backslider need not despair of again finding mercy; this passage holds out sufficient encouragement for his hope. There is scarcely another such in the Bible, and why? That sinners might not presume on the mercy of God. And why this one? That no backslider might utterly despair. Here, then, is a guard against presumption on the one hand, and despondency on the other.

Clarke Commentary