There is a story that when Sir Walter Scott lay dying, he asked his friends to bring him the book and read it to him, as he was in the last minutes of his life. His friends replied: "Sir since you have so many books, which book you want us to bring?" "There is only one book," he replied, "and that is the Holy Bible."
In a sense, there has been a dilution of the term that Scott has used. At one point it was understood that Scriptures meant only one book - the Holy Bible. It is possible today to read various press releases and see the term "scriptures" or "holy writings" applied to the Hindu writings, the Koran or the Christian Bible.
This dilution of terms reminds me of the famous quote by E.E. Cairns: "If you do not believe anything, you can say everything." In many books on comparative religions, the Christians who write these books have given up on the authenticity and the uniqueness of the Holy Bible, as they emphasize that each religion has its own sacred texts.
I want to approach this topic from two angles - a) the veracity or truthfulness of the sayings of Jesus, and b) the law of contradiction (also known as the law of non-contradiction).
Jesus challenges his Jewish audience to read the Scriptures because in them they will find the truth the Scriptures tell about Him. Later on, we find out that the gospels, and especially the Gospel of Matthew, which is written to the Jews and seeks to demonstrate that Jesus is the fulfillment of the Old Testament. Apostle Paul defines the Gospel as Christ who was crucified, buried, and raised on the third day, according to the Scriptures.
The second thing has to do with the Jesus asking His disciples to tell who he is. In that particular encounter, recorded in every gospel, Pete makes that confession on which the church stands: "You are Christ, the Son of God."
Many people tell us that the Koran talks about Jesus Christ, and that is true. We find out that there are numerous passages about Jesus Christ. It is a very impressive record. However, this Jesus has very little similarity to the Jesus of the New Testament. While he is a miracle worker, he is not divine and he did not die on the cross. If Jesus is not divine he could not have redeemed us, and during his whole life he pointed out that he was going to die on the cross, for the sins of humanity.
The Jesus of the Koran cannot save. In fact, in the hierarchy of prophets, Mohammed is higher than Jesus Christ. Jesus is not one with the Father, Jesus did not die to save sinners, and Jesus will not judge the world. In fact, the uniqueness of Jesus as the Son of God, of the same essence with God the Father, is blasphemy for the Koran.
The law of contradiction (or non-contradiction) states that a cannot be a non-a at the same time or a cannot be b unless b is equal to a. The law of contradiction (or non-contradiction) is that one of these texts must be false. You cannot have the New Testament and the Koran true at the same time. If Jesus is as the New Testament tells us, he cannot be as the Koran tells us. If he is what the Koran tells us, then he cannot save us and we should all become Muslims because Christ is not needed for the salvation of humanity.
If the New Testament is true, then its inspiration is true. If the inspiration of the New Testament is true, then the inspiration of the Koran is not true. If the New Testament inspiration is of God, and God does not contradict himself in revelation, then the revelation of the Koran is not from God.
Nicky Gumbel in one of his presentations tells us about a little boy who after his geography test prayed, "Lord, be merciful and just for once, let Paris be the capital city of Turkey." Even God cannot do that because that is a contradiction. If we work with a precise definition of the inspiration and the canonization of the New Testament, then we have to say that after the New Testament was closed, divine revelation to become a part of the New Testament ceased. Based on this definition, the Koran and the Book of Mormons do not qualify as being inspired by God.