The word propitiation occurs only here and in chap. 4 throughout the New Testament: it is really the counterpart of the ‘blood of Jesus His Son’ in chap. 1 John 1:6, the administration of the atonement coming between them in chap. 1 John 1:9. Christ is in the New Testament ‘set forth as a propitiation in His blood’ (Romans 3:25): a sacrificial offering that, as on the day of atonement to which it refers, averted the wrath of God from the people. He also as High Priest made atonement or ‘propitiation for the sins of the people’ (Hebrews 2:17). which is here, as in the Septuagint, ‘propitiated in the matter of sins’ the God of holiness. Uniting these, He is in the present passage Himself the abstract ‘propitiation’ in His own glorified Person. His prayer for us, issuing from the very treasure-house of atoning virtue, must be acceptable; and, uttered to the Father who ‘sent Him’ as the propitiation (chap. 1 John 4:14), is one that He ‘heareth always’ (John 11:42).
It is then added: and not for ours only, but also for the whole world. And why? First, because the apostle would utter his generous testimony, on this his first mention of the world, to the absolute universality of the design of the mission of the ‘Lamb of God who taketh away the sin of the world:’ his last mention of it, the second time he says ‘the whole world,’ will be of a severer character (chap. 1 John 5:19). Secondly, he thus intimates that the proper propitiation, as such, was the reconciliation of the Divine holiness and love in respect to all sins at once and in their unity, while the advocacy based upon it refers to special sins: on the one hand, no other atonement is necessary; on the other, that must avail if penitence secures the advocacy of Him who offered it once for all. Lastly, as we doubt not, the apostle thus ends a discussion, the fundamental object of which was to set forth universally and in general the way in which the Gospel offers to all mankind fellowship with the light of God’s holiness.
Schaff, Philip. "Commentary on 1 John 2:4". "Schaff's Popular Commentary on the New Testament". https://www.studylight.org/commentaries/scn/1-john-2.html. 1879-90