Theophylact of Ohrid — as excerpted in the Catena Aurea on John 3:16-18

“(in loc.) As He said above, that the Son of man came down from heaven, not meaning that His flesh did come down from heaven, on account of the unity of person in Christ, attributing to man what belonged to God: so now conversely what belongs to man, he assigns to God the Word. The Son of God was impassible; but being one in respect of person with man, who was passible, the Son is said to be given up to death; inasmuch as He truly suffered, not in His own nature, but in His own flesh. From this death follows an exceeding great and incomprehensible benefit: viz. that whosoever believeth in Him should not perish, but have everlasting life. The Old Testament promised to those who obeyed it, length of days: the Gospel promises life eternal, and imperishable.”