And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement. We also joy (καυχωμενοι, we exult, or glory) in God, etc. - We now feel that God is reconciled to us, and we are reconciled to him: the enmity is removed from our souls; and He, for Christ's sake, through whom we have received the atonement, καταλλαγην, the reconciliation, has remitted the wrath, the punishment which we deserved: and now, through this reconciliation, we expect an eternal glory.
It was certainly improper to translate καταλλαγη here by atonement, instead of reconciliation; as καταλλασσω signifies to reconcile, and is so rendered by our translators in all the places where it occurs. It does not mean the atonement here, as we generally understand that word, viz. the sacrificial death of Christ; but rather the effect of that atonement, the removal of the enmity, and by this, the change of our condition and state; from κατα, intensive, and αλλασσω to change; the thorough change of our state from enmity to friendship. God is reconciled to us, and we are reconciled to him by the death of his Son; and thus there is a glorious change from enmity to friendship; and we can exult in God through our Lord Jesus Christ, by whom we have received this reconciliation. Though boasting is forbidden to a Jew, because his is a false confidence, yet boasting is enjoined to a Christian, to one reconciled to God; for, his boasting is only in that reconciliation, and the endless mercy by which it is procured. So he that glorieth (boasteth) must glory in the Lord.