I want to say up front that this isn't original to me. I read it somewhere, and I'd love to give them credit, but I can't find where I saw it. But it was interesting, so I wanted to share it here.
The basic question is this: When God became a man to die on the cross, did it have to be the Son? Could the Father or the Spirit have been incarnated instead?
It's something to neat ponder, but it sounds like it might be a little too close to asking how many angels can stand on the head of a pin. The answer turns out to be much more profound than that, though.
The answer hinges on this: Why the cross?
Jesus died on the cross to save us from our sins. He was crucified so that we could be justified, made right with God.
But that's not the only reason. It wasn't even the primary reason.
Saving us from our sins was a means to an end. It had to be done so that something else could be achieved:
"For those God foreknew he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brothers and sisters" (Rom 8:29).
"For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he predestined us for adoption to sonship through Jesus Christ ..." (Eph 1:4-5).
This was the plan from the beginning: We were saved so that we could be adopted. Our sins are forgiven so that we can become children of God. We are made one with Christ so that we can share in his inheritance. We were not saved so that we could be servants or even courtiers. We are the children of the King.
The relationship we were meant to have with God was meant to be like that of the Father and the Son. So neither the Father nor the Spirit could have filled the role that the Son did.