Theologically, why was it necessary that Jesus be perfect? Couldn’t there be another ordinary human who said “I will be the sacrifice and take on all the sins of the world past, present, and future and suffer and die as a substitutionary atonement for all of mankind”? Why did it have to be Jesus and why did it have to be a perfect Jesus?
Fallen human beings are all disqualified because we all inherit the sinful nature.
If Jesus had even one sin, He could never die for others' sins because He would have to die for His own. Therefore Christ knew no sin (2 Cor. 5:21; Heb. 4:15). He was made only in the likeness of the flesh of sin (Rom. 8:3). He was without spot or blemish (1 Pet. 1:19).
Him who knew no sin he made to be sin on our behalf; that we might become the righteousness of God in him (2 Cor. 5:21). For we have not a high priest that cannot be touched with the feeling of our infirmities; but one that hath been in all points tempted like as we are, yet without sin. (Heb. 4:15)
Yet if Jesus were only a sinless man, we would still have a great problem. His death would be good for only one man; in fact, it would be good for only one sin. God requires man to die because of sin—even one sin. Suppose in your entire life you committed only one sin. As a sinless man, Jesus could die as a substitute for that sin. But if you were to sin again you would have to die for that second sin. Well, you know that you have sinned more than once in your life! How could one man, Jesus, die for all your sins and for all the sins of mankind? And how could it work for us two thousand years after His death?
He was a genuine man, yet also the complete God. His being God added an eternal element to His redeeming blood. Hebrews 9:14 says that the Lord offered Himself up through the eternal Spirit. He has accomplished an eternal redemption for us (Heb. 9:12). Now Jesus' blood is effective for all men of all times. It can cleanse every sin of every man on the earth throughout time and space.
12 nor yet through the blood of goats and calves, but through his own blood, entered in once for all into the holy place, having obtained eternal redemption.
14 how much more shall the blood of Christ, who through the eternal Spirit offered himself without blemish unto God, cleanse your conscience from dead works to serve the living God?
According to the levitical priesthood a sacrifice had to be without defect
Lev 22:20 Do not bring anything with a defect, because it will not be accepted on your behalf.
In this case any sacrifice offered to God has to be without defect. We humans are born into sin and no one lives a life without sin. Romans 3:23 tells us we have all sinned and fallen short of the glory of God. Sin is bad because it separates us from God.
Isaiah 59:2 But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear.
So ideally God would not accept any sacrifice by any man since sin has already made them impure. The one person who lived without sin was Jesus so He was the only one capable of being offered as an acceptable sacrifice
I'd just add that anyone other than a perfect sacrifice (Jesus) would also be tainted with sin and would require a savior themselves. How could any sinful person provide salvation for others when they themselves need saved from their own sinful condition?
The factor which makes a sacrifice acceptable to God has to do with obedience to the original command of God not to eat of the tree of the knowledge of good and evil.
When Adam and Eve ate from the tree of the knowledge of good and evil, their souls were contaminated in two ways, which made them no longer acceptable to God.
- They disobeyed God.
Matthew 10:28 KJV And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
- Satan usurped The position of God as the main influence in their lives.
Genesis 3:1 through 5 KJV Now the serpent was more subtil than any beast of the field which the LORD God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden? 2 And the woman said unto the serpent, We may eat of the fruit of the trees of the garden: 3 But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die. 4 And the serpent said unto the woman, Ye shall not surely die: 5 For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.
Eve believed Satan more than she regarded God's command not to eat. When Adam ate he was not deceived but obeyed not only Satan but also Eve who:
Genesis 3:6 KJV And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.
Adam knew that he was disobeying God; but ate disregarding God.
Once their souls were contaminated their progeny (us) were no longer acceptable since impure cannot begat pure.
Hope this helps.