First, the divine nature is altogether incapable of being transformed. Second, because such unions involve a transformation of the united elements, the result would be that Christ was neither divine nor human, which is clearly heretical. Third, this sort of mutually transforming union cannot take place when one of the items is divinity: somewhat as putting a drop of water into a large vessel of wine results (Aquinas says) in just plain wine, so too would mixing humanity with divinity result in something that was entirely divine, again giving us a heretical result.
