Luke 1:35, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God.”
The incarnation reveals the Trinity. Yes, when the Word takes on flesh, He does not only reveal the Father, but He reveals the differentiated unity of God’s being. He reveals the inner life of God; the Father generating the Son in His own image, the Son generated by and obedient in love to the Father, the Spirit as the personal medium of the life of Father and Son; as the love between them (Luke 3:22). We know God as Trinity because the Spirit overshadowed Mary in the power of the Most High so that the Son of God became flesh and was born into the world.
Now, one question that might be raised is this: if the economic Trinity perfectly reveals the immanent Trinity—specifically in that the roles of the Persons within history reveal something of their eternal role within the Godhead—then what are we to make of the Spirit being the agent of the Son’s conception? If the Son is the one to be conceived and born due to His role as the eternally generated image of the Father, does the economic role of the Spirit explained in the annunciation not seem imply that the Spirit is the agent of the Son’s eternal generation within the Immanent Trinity?
A response to this might be that the Spirit’s role in the Son’s incarnation does not depict Him as the agent of eternal generation, but as the agent or medium of the Son’s obedience. In this sense, the incarnation is viewed less as an image of the Son’s eternal generation and more as the supreme instance of His loving obedience, which certainly has more biblical support (John 5:20ff, Phil 2:6-9, etc). In this view, then, the Spirit’s role in the incarnation is analogous to His role in the crucifixion. Just as the Spirit is the personal medium through whom the Son offers Himself up in love-borne obedience to the Father (Heb.9:14), so too at the incarnation the Spirit is the personal medium through whom the Son begins that journey of perfect obedience, making Himself nothing, taking on the form of a servant.
