Theophylact of Ohrid

To the words: "from God the Father and from the Lord Jesus Christ," he added also: "the Son of the Father." For God alone is Father of the Son in the proper sense. Therefore Paul also says: "from Whom every fatherhood in heaven and on earth is named" (Eph. 3:15). In truth and love. With these words he lends credibility to the speech and points out the mark of the love of which he speaks. "Peace" — that is, peace that is true and firm, and not in appearance only.

Commentary on 2 John ↗

Oecumenius

Again, after these things, he adds: "Grace and mercy will be with us:" designating the goods that arise from perfect love. Speaking of God the Father and from the Lord Jesus Christ, John adds, "the Son of the Father:" for only here is He properly the Father. Therefore, Paul also says, "From whom," he says, "every family in heaven and on earth is named."(Eph. 3:15) He further adds: "in truth and love," bringing firmness to the discourse, and a certain indication of the love that he speaks of.

Commentary on 2 John ↗

Bede

May grace, mercy, and peace from God the Father be with you, etc. Since the heretics of that time, namely Marcian and Cerinthus, denied that our Lord Jesus Christ was truly the Son of God and attributed his beginning to human birth, John rightly recalls him as the Son of God the Father to refute these blasphemers. …

Commentary on the Catholic Epistles ↗