From that opening of the heavens, a loud voice was heard, that its majesty might be more impressive. The public appearance of Christ, to undertake the office of Mediator, was accompanied by this announcement, that we may rely on this pledge of our adoption, and boldly call God himself our Father.
The designation of Son belongs truly and naturally to Christ alone: but yet he was declared to be the Son of God in our flesh, that the favor of Him may be also obtained for us. Thus when God presents Christ to us as Mediator, accompanied by the title of Son, he declares that he is the Father of us all, (Ephesians 4:6.)
These words, "in whom I am well pleased," imply that the love of God rests on Christ in such a manner, that it can diffuse itself from him to us all.
[Excerpt from Calvin's Commentaries]