Christians are limiting God by saying God can't simply forgive out of mercy, but he *needs/requires* punishment/justice to let go of the sinners.
I haven’t really thought about this issue too much before, but I’d imagine that this could have serious consequences for God’s abilities, powers, and aspects of his benevolent nature if true. If you believe that God demands justice and is truly fair, His mercy seems like it does not fit into the picture.
To put it in a logical and readable sequence, I have laid out what I think your argument might be below:
1. If God requires perfect justice, then it is necessary that sinners cannot be free without judgment
2. If it is necessary that sinners cannot escape judgment, then God cannot simply forgive these sinners only out of His free will (i.e. forgiveness through requiring the punishment of Jesus)
3. It follows that if God requires perfect justice, then God cannot forgive sinners out of His free will(1 and 2 HS )
4. God requires perfect justice
5. God cannot forgive sinners out of his free will(3 and 4 MP)
6. If God cannot forgive sinners out of free will, then He is limited in his abilities
7. Therefore, God is limited in his power to forgive out of His free will (5 and 6 MP)
I believe that there may be an issue with premise 2 and premise 6.
Against premise 2, I believe that God can offer mercy and justice simultaneously. The death, burial, and resurrection of Jesus is both justice and mercy at the same time. It still is full justice due to the price of sin being paid in full when Jesus was crucified. However, it is also merciful in the fact that those who sin are offered divine mercy. The sinners receive mercy while the guilt of sin is accounted for. It is the sin that God hates and seeks atonement for, but not the sinners in and of themselves.
Against premise 6, due to the nature that God is perfectly just, His inability to let those sins go unpaid for is not a limitation of God’s greatness or power. If God left sin go unpunished and unaccounted for, this would signal a fault in His system of justice. This limitation is not a weakness, but an absence of a weakness. Similarly, God cannot be evil or weak, but that does not speak against his greatness. A limitation of how weak God is only speaks to His strength.