"IT IS written (Jn 3:14): "As Moses lifted up the serpent in the desert, so must the Son of man be lifted up, that whosoever believeth in Him may not perish, but may have life everlasting."
"...As the Philosopher teaches (Met. v), there are several acceptations of the word "necessary." In one way it means anything which of its nature cannot be otherwise; and in this way it is evident that it was not necessary either on the part of God or on the part of man for Christ to suffer. In another sense a thing may be necessary from some cause quite apart from itself; and should this be either an efficient or a moving cause then it brings about the necessity of compulsion; as, for instance, when a man cannot get away owing to the violence of someone else holding him. But if the external factor which induces necessity be an end, then it will be said to be necessary from presupposing such end—namely, when some particular end cannot exist at all, or not conveniently, except such end be presupposed. It was not necessary, then, for Christ to suffer from necessity of compulsion, either on God's part, who ruled that Christ should suffer, or on Christ's own part, who suffered voluntarily. Yet it was necessary from necessity of the end proposed; and this can be accepted in three ways. First of all, on our part, who have been delivered by His Passion, according to John (3:14): "The Son of man must be lifted up, that whosoever believeth in Him may not perish, but may have life everlasting."
"Secondly, on Christ's part, who merited the glory of being exalted, through the lowliness of His Passion: and to this must be referred Luke 24:26: "Ought not Christ to have suffered these things, and so to enter into His glory?"
"Thirdly, on God's part, whose determination regarding the Passion of Christ, foretold in the Scriptures and prefigured in the observances of the Old Testament, had to be fulfilled. And this is what St. Luke says (22:22): "The Son of man indeed goeth, according to that which is determined"; and (Lk 24:44-46): "These are the words which I spoke to you while I was yet with you, that all things must needs be fulfilled which are written in the law of Moses, and in the prophets, and in the psalms concerning Me: for it is thus written, and thus it behooved Christ to suffer, and to rise again from the dead.""
~St. Thomas Aquinas: Summa Theologiae, III, q. 46, a. 1.
• See this and related questions in the Summa here.
Artwork: Crucifixion (detail), by Andrea da Firenze. Fresco, 1366-67; Cappellone degli Spagnoli, Santa Maria Novella, Florence.
