Objection 1. It would seem that it was not necessary for Christ to suffer for the deliverance of the human race. For the human race could not be delivered except by God, according to Isaiah 45:21: "Am not I the Lord, and there is no God else besides Me? A just God and a Saviour, there is none besides Me." But no necessity can compel God, for this would be repugnant to His omnipotence. Therefore it was not necessary for Christ to suffer.
Objection 2. Further, what is necessary is opposed to what is voluntary. But Christ suffered of His own will; for it is written (Isaiah 53:7): "He was offered because it was His own will." Therefore it was not necessary for Him to suffer.
Objection 3. Further, as is written (Psalm 24:10): "All the ways of the Lord are mercy and truth." But it does not seem necessary that He should suffer on the part of the Divine mercy, which, as it bestows gifts freely, so it appears to condone debts without satisfaction: nor, again, on the part of Divine justice, according to which man had deserved everlasting condemnation. Therefore it does not seem necessary that Christ should have suffered for man's deliverance.
Objection 4. Further, the angelic nature is more excellent than the human, as appears from Dionysius (Div. Nom. iv). But Christ did not suffer to repair the angelic nature which had sinned. Therefore, apparently, neither was it necessary for Him to suffer for the salvation of the human race.
On the contrary, It is written (John 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."
I answer that, As the Philosopher teaches (Metaph. 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 (Luke 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."
Reply to Objection 1. This argument is based on the necessity of compulsion on God's part.
Reply to Objection 2. This argument rests on the necessity of compulsion on the part of the man Christ.
Reply to Objection 3. That man should be delivered by Christ's Passion was in keeping with both His mercy and His justice. With His justice, because by His Passion Christ made satisfaction for the sin of the human race; and so man was set free by Christ's justice: and with His mercy, for since man of himself could not satisfy for the sin of all human nature, as was said above (Question 1, Article 2), God gave him His Son to satisfy for him, according to Romans 3:24-25: "Being justified freely by His grace, through the redemption that is in Christ Jesus, whom God hath proposed to be a propitiation, through faith in His blood." And this came of more copious mercy than if He had forgiven sins without satisfaction. Hence it is said (Ephesians 2:4): "God, who is rich in mercy, for His exceeding charity wherewith He loved us, even when we were dead in sins, hath quickened us together in Christ."
Reply to Objection 4. The sin of the angels was irreparable; not so the sin of the first man (I, 64, 2).
Source: Summa Theologica (New Advent)