If a person commits a sin such as murder or theft, should that person be punished? The obvious answer is yes. But, why should he be punished?
There are two primary reasons we can offer why someone who commits a sin should be punished. First, he has broken the law of God; and if God does not exercise punishment, then what is the purpose of the law? To not exercise punishment for committing sin is to permit the sin to exist without consequence. Second, he has harmed others; and if he is not punished, others will be encouraged to do the same.
Therefore, we can conclude that it is proper to exercise judgment/punishment for the crimes committed by individuals.
If the same person who has committed a sin, such as murder or theft, says that he is sincerely sorry for what he has done and that he would never do it again, should society excuse him from punishment? Maybe, maybe not.
God is holy and righteous; and if he didn't carry a punishment for sins, he would be allowing evil to exist without consequence. The result would be, essentially, to condone evil; and since God cannot do that, His justice requires that a proper punishment be incurred for each sin.
Likewise, if God did not punish someone because that person said he was sorry and would not do it again, then is God being just? After all, if he did not exercise the punishment for breaking his law, he is then permitting evil to exist without proclaiming its error or dealing with its consequence.
In Islam, a Muslim who commits murder and thievery can ask Allah for forgiveness and receive that forgiveness without Allah exercising any punishment whatsoever. There are undoubtedly situations where Muslims have committed grievous sins, have not been caught, and have later sincerely repented before their God, have asked forgiveness, and (theoretically) been forgiven. We ask, where is Allah's exercise of judgment upon the sin that was committed?
According to Islam, Allah may forgive that person--if he so chooses. But, doesn't that mean the righteous judgment of Allah is not satisfied? Doesn't it mean that the sin has essentially been ignored by forgiving it; and that the Law of God, though broken, has resulted in no punishment? Is this just--not to exercise punishment for the sins committed? It is not. No, for we have already established that punishment is the proper and righteous response to sin. Declining to punish is to avoid satisfying the law of God.
Therefore, since we can rationally propose a situation where the God of Islam will forgive a Muslim without there being punishment according to law, we can conclude that the God of Islam is unjust. If Allah is unjust, then he is not the true God.
In Christianity, every sin is dealt with by God in one of two ways. Either God satisfies the law by exercising his punishment upon the sinner by sending him to eternal damnation, or, he places the sins of the person on Jesus Christ who suffers the punishment in place of the person. Either way, the justice of God is never ignored. It is proper that sin be dealt with by a punishment relative to the sin. To neglect to do so is to avoid being just.
Therefore, we see that the God of Christianity is just because no sin goes unpunished. After all, the Bible describes God as being holy and righteous. His standard of holiness is so great that all sin must be dealt with properly. The Law, which is a reflection of God's holy character and nature, is not to be violated without consequence. The consequence of all sin is death.
The only way for us to escape the righteous and holy judgment of God is to receive the sacrifice of Christ made on our behalf where he bore our sins in his body on the cross (1 Pet. 2:24). There is no other way . . . except, that is, to pay for your sins yourself in hell forever.
First: This means that the Christian is escaping judgment. This is true (Christianity and Islam teach this). But it is also true that the sin is being addressed in the person of Christ, and the punishment of that sin is satisfied--unlike Islam where the proper punishment due for breaking the law is not met. Furthermore, in Christianity, forgiveness of sins is assured by the sacrifice of Christ who bore our sins in his body on the cross, which is why we do not have to earn our forgiveness in any way.
"and He Himself bore our sins in His body on the cross, that we might die to sin and live to righteousness; for by His wounds you were healed." (1 Pet. 2:24).
Second: Is it just to transfer the punishment from one person to another? Yes, since the law is still satisfied in that breaking it is dealt with. In Christianity, the debt, the penalty incurred by sin, was transferred to Christ; and the punishment of God fell upon the person of Christ on the cross.
"And when you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, 14having canceled out the certificate of debt consisting of decrees against us and which was hostile to us; and He has taken it out of the way, having nailed it to the cross." (Col. 2:13-14).
Therefore, all who trust in Christ for the forgiveness of their sins are trusting in the sacrifice of Christ, who fell under the wrath of God the Father and was punished in their place. In this way, sin is dealt with by an act of justice. Remember, a holy and righteous God cannot let sin go unpunished. We must ask then, why does the God of Islam not do what is just?