The hated Nestorius infamously, impiously stated that he would not worship a God who was once a baby (or some such nonsense; I don't have the exact quote on hand). It is this kind of errant thinking that is denied by the use of the term Theotokos, affirming instead that, yes, St. Mary truly gave birth to
God, and not a mere human man named Jesus who, by virtue of being the Son of the Father, would therefore be considered God by that connection (as explicitly condemned in the twelve anathemas of St. Cyril which were affirmed at the same council in Ephesus). Nestorianism preaches a radical separation between God and Christ such that saying Theotokos is seen as inappropriate, because St. Mary is only the source of Jesus' human nature and flesh, and not of His divinity. So of course they would no doubt say that Jesus can be a baby because every human goes through that stage, and Jesus is a human person, but at the same time it inappropriate to say that of divinity because God was not running around in a first-century diaper, suckling from His mother, etc. Only the human being Jesus was doing that, they say.
Yet everyone here says otherwise, and rightly so, because Christ is perfectly and completely God
and man,
not according to what activity we may be talking about or what human age at any given point, but because He is the Son of God and the son of St. Mary, the Theotokos!
(I'm just gonna leave this Eastern Orthodox icon here, in case any Nestorians happen to drop by and need the point underlined.

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