If it is true that all three divine persons are involved any act of God, then the incarnation of the Son involved the whole Trinity. That does not mean that the Father and the Holy Spirit as well as the Son became incarnate. It was fitting that the Son as the image of the invisible God became man, created in the image of God according to his human nature.
But the Son did not become incarnate apart from the Father and the Holy Spirit. The Father sent the Son into the world as man and sustained, taught, guided and empowered him by the presence of the Holy Spirit. The purpose of the incarnation was that Christ might redeem us from sin by offering himself without blemish to God through the eternal Spirit. The Father raised his Son from the dead by the Spirit of holiness and by that same Spirit exalted Christ to his right hand in glory. The glorified Jesus poured out the Spirit from the Father upon the church on the Day of Pentecost.
Through the Son the godhead bears the impress of Jesus’ human experience of humiliation, sorrow and death. Our triune Lord remembers our frame, knowing that we are dust. The Holy Spirit as the Spirit of Christ, who groaned under the burden of our fallen world, makes intercession for us with groanings that cannot be uttered. The Father of the suffering Son’s ear is ever attentive to the cries of his children. His tender hand will wipe all tears from their eyes.
Believers will share in the glory and exaltation of the incarnate Son. He, as the life-giving Spirit will raise them from the dead, transform them into his image and welcome them into his Father’s house. There they will dwell with the Triune God for ever. Christmas is the gift of the Trinity.