(Rank amateur alert: Read at your own risk.)
God cannot be an eternal Lover without an eternally Beloved. When God speaks to himself, he is the Speaker, the Listener, and the Conversation. God cannot be self-conscious unless he is both the observer of God, the God being the observed, and the act of Observation. ie. Christ is the image of God reflected in the mirror of God’s mind, and the light flashing between (as it were) is the Holy Spirit. (To use a physics metaphor, the Spirit is the exchange particle, binding God together.)
Christ doesn’t exist (or pre-exist) in the sense that things exist. He is no thing. He is the source of all things.
I am a non-material mind living in intimate relation to a material body. I am also the mysterious something that mediates between body and mind. Suppose I think a non-material thought: a circle. This thought is somehow mediated to my muscles, and I draw said circle on a piece of paper. A real thought has been made real in the material world. It has moved from mind-space into material-space. The thought is not a muscle. The mediator that links the thought to my muscles is neither thought nor muscle, but something else again. The thought, the mediator and the muscle are all different, and they’re all me. An analogue, perhaps, of the Trinity.
In the same way, Christ mediates the mind of God to the material world. Christ speaks the words given him by the Father. This is his role. He makes God’s thoughts materially real. “By him were all things made.” He always has and always will. This suggests the material universe is an infinite and eternal creation. (According to Aquinas, this notion doesn’t challenge Genesis etc .)