Question: If for the reasons given, then, the only way to eternal life is the crucified Jesus, truly God and truly human, does it necessarily follow that all other religions or faiths, which present other ways and means to eternal life, are false?
Answer: Only the Christian religion and faith is the true faith; all others are false. For only Christians recognize God as one true God, who is perfectly just and perfectly merciful and thus the true God. He is perfectly just in that He does not leave sin unpunished but punished each and every sin with unspeakable hellish torment in His Son on the wood of the cross, whereby not a half but a full and just payment for our sins was made. He is perfectly merciful in that He makes us pay absolutely nothing but out of sheer mercy gave us the Son for our payment, without any merit on our part, while we were yet sinners.
By contrast, all other religions and faiths do not recognize God as perfectly just and merciful. Think once of the Jews, the Turks, or the Papists, who know by nature that God is just and will not leave sin unpunished. Yet, when their consciences bother them, they come and offer a gift upon the altar, hoping thereby to buy off God for sins that deserve eternal damnation. That is just like a thief or murderer who, in trying to buy off the judge with money or a gift, shows that he does not consider the judge to be just; otherwise he would not expect to be able to bribe him. Since, then, religions other than Christianity cannot bring to the judgment seat of God a full payment (which only Christ can do) but only a few trifling merits and tattered works, not even a thousandth the size of the sin they wish to pay with them, it is certain that these other religions do not regard God as perfectly just.
By the same token, they do not regard Him as perfectly merciful either, for they think that they have already made half the payment with their offerings and worship. But God is not half just and half merciful; He is completely so and wishes to be regarded and recognized as such. Therefore, all other religions, however many there are, in fact rob God of His perfect justice and mercy, because they worship not the true God but an idol that they themselves have conjured up and that is neither perfectly just nor perfectly merciful — in short, not the true God.
Caspar Olevianus, A Firm Foundation, pp. 7, 8