I’ve written before on Calvin’s extra – his view that during the incarnation God the Son still filled the heavens and upheld all things as Infinite God. Before Calvin, this view was asserted by Athanasius in On the Incarnation of the Word, III.17:
The Word was not hedged in by His body, nor did His presence in the body prevent His being present elsewhere as well. When He moved His body He did not cease also to direct the universe by His Mind and might. No. The marvelous truth is that being the Word, so far from being Himself contained by anything, He actually contained all things Himself. In creation He is present everywhere, yet is distinct in being from it; ordering, directing, giving life to all, containing all, yet is He Himself the Uncontained, existing solely in His Father…. His body was for Him not a limitation, but an instrument, so that He was both in it and in all things, and outside all things, resting in the Father alone. At one and the same time – this is the wonder – as Man He was living a human life, and as Word He was sustaining the life of the universe, and as Son He was in constant union with the Father.
The other pre-reformation text I’ve read so far is Basil the Great’s On the Holy Spirit, and I was interested to see that Basil also affirms the extra-Calvinisticum in 8.18: “the Infinite God, remaining changeless, assumed flesh and fought with death, freeing us from all suffering by his own suffering” (italics mine).
This is really an amazing thought, that the Son of God was, as one person, simultaneously sleeping in a manger and upholding the universe. But as I have argued before, I think a robust Creator-creation distinction absolves any difficulties that arise.