“God made him who had no sin to be sin[a] for us, so that in him we might become the righteousness of God” (2 Cor 5:21).
The [a] is a footnote in the NIV which says “Or be a sin offering.”
There’s a big difference between becoming sin and being a sin offering.
Did Jesus literally become sin when he was on the cross? Or does the verse mean that if we have turned from our sin and begun to trust and obey Jesus (because of his death), it is as though our sins were crucified on the cross?
In the Old Testament the priestly sacrifice of a lamb, bull or goat was meant to bring about repentance and consequently obedience. Without repentance the sacrifice had no power to justify the sinner.
“Does the Lord delight in burnt offerings and sacrifices
as much as in obeying the Lord?
To obey is better than sacrifice,
and to heed is better than the fat of rams” (1 Sam 15:22).
God delighted more in obedience than he delighted in the sacrifice. (His delight in the sacrifice was because it was an act of obedience.)
It’s was Jesus obedience that delighted the Father, not his death on the cross.* And if we do not turn and trust and obey Jesus because of his death on the cross, the Father will not cancel our debt.
Without repentance, Jesus death on the cross does no more good for a sinner than the sacrifice of bulls for unrepentant Israel.
“The multitude of your sacrifices—
what are they to me?” says the Lord.
“I have more than enough of burnt offerings,
of rams and the fat of fattened animals;
I have no pleasure
in the blood of bulls and lambs and goats.
12 When you come to appear before me,
who has asked this of you,
this trampling of my courts?
13 Stop bringing meaningless offerings!
Your incense is detestable to me.
New Moons, Sabbaths and convocations—
I cannot bear your worthless assemblies.
14 Your New Moon feasts and your appointed festivals
I hate with all my being.
They have become a burden to me;
I am weary of bearing them.
15 When you spread out your hands in prayer,
I hide my eyes from you;
even when you offer many prayers,
I am not listening.Your hands are full of blood!
16 Wash and make yourselves clean.
Take your evil deeds out of my sight;
stop doing wrong.
17 Learn to do right; seek justice.
Defend the oppressed.[a]
Take up the cause of the fatherless;
plead the case of the widow.18 “Come now, let us settle the matter,”
says the Lord.
“Though your sins are like scarlet,
they shall be as white as snow;
though they are red as crimson,
they shall be like wool.
19 If you are willing and obedient,
you will eat the good things of the land;
20 but if you resist and rebel,
you will be devoured by the sword.”
For the mouth of the Lord has spoken (Isaiah 1:11-20 See also Hos 6:6; Psalm 40:7, 8; Psalm 51:16-17).
A person’s sacrifice in the Old Testament was meant to show that they were repentant. If they were unrepentant, their sacrifice was not pleasing to God. Their sacrifices were not really the problem; the problem was their unrepentant hearts. That's what sickened God. That's what made their sacrifices detestable.
If we claim to be forgiven because of Jesus death, while continuing to disobey him, we sicken God.
Jesus died so that we might be drawn to him and begin to trust and obey him. Jesus death on the cross is meant to lead to our repentance. He did not literally become sin on the cross. He became a sin offering. An offering which is only of value to us if we turn to him and repent. We do not have to be perfect to please God, but we do have to trust and obey him.
In the Old Testament it was repentance that led to forgiveness. (There are examples of people being forgiven in the OT without a sacrifice being offered. See here.)
The way to God has never changed. It has always been through trust and obedience. Jesus death on the cross didn’t change that. Jesus lived, and died, and rose from the dead, and lives again so that we might turn to him and trust his Father. The only real difference between the old covenant and the new is now trusting and obeying God means trusting and obeying the Messiah whom he has sent. If you do not obey the Messiah, you are disobeying God.
God knows we will not obey perfectly, but he does not require perfect obedience in order for him to forgive. All he wants from us is to trust him. If we are trusting him he is pleased with us. Then he will begin to save us from sin and start making us like Christ.
If this has raised some theological difficulties for you, you might find the following video helpful.
Do we have to be perfect to be accepted by God?
What's Wrong with the Innocent Dying for the Guilty?
Penal Substitutionary Atonement
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*See Ezek 18:32 , John 8:29 and Phil 2:8