For there is one God and one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all men – the testimony given in its proper time. (1 Timothy 2:5-6)
The message of Christianity is that there is only one way to know God, to gain truth, and to enter heaven, and that is our way, the way of Jesus Christ. All other religions and philosophies are false, and lead men to delusion and everlasting punishment. Because of this, all people can be divided into Christians and non-Christians, and all thinking can be divided into Christian views and non-Christian views. Although there are varieties of non-Christians, they are essentially the same, having in common their basic principles of thought, their unrighteousness and rebellion, their irrational and foolish thinking, and their ultimate damnation. On the other hand, there are no such shared characteristics between what is Christian and what is non-Christian. This is the distinction that truly makes the difference.
It is often said that the three major monotheistic religions are Judaism, Christianity, and Islam. This could be true under a tremendously broad definition of God, but it is misleading in serious religious discussions and comparisons.
Christians affirm that there is only one God, and he is a Trinity, and that there is no God that is not a Trinity. An adequate consideration of any member of the Trinity must include his relationship to the other members. Thus, for example, an adequate consideration of the second member must mention that he is the Son; however, this necessarily denotes a relationship to the Father. The significance of this point is that, although we affirm that each member of the Trinity is fully God in the sense that each possesses all the attributes and powers of God (each is all-powerful, all-knowing, etc.), the Triune nature of God is an integral part of the definition of God, so that the Son is God not only because he is all-powerful, all-knowing, and so on, but also because he has a place in the Trinity. Therefore, there is only one God in the sense that there is only one Trinity.
This is surely unacceptable to the false religion of Islam. Beginning from their erroneous idea of God, Muslims cannot, or at least should not, acknowledge that Christianity is monotheistic in the same sense or in a sense similar enough that it remains meaningful to think that the two religions have this in common.
As for Judaism, the Trinity is consistent with God’s revelation in the Old Testament. God has revealed himself as a family or a community since the beginning, and as the history of revelation progressed, this was made known with increasingly explicit terms. Yet the Jews think that the Christian concept of God is blasphemous. Why? Jesus said that the Jews did not believe the Scripture. They had never accepted what God said about himself. Even Christians might hesitate to say that the Jews affirm some different idea of God, but the truth is plainly recorded. When God revealed himself to them through intelligent speech without bodily form, they made a golden calf. And when he appeared to them in human form to teach them about himself, they killed him.
Jesus said that if they had believed the Old Testament, then they would have recognized him and believed him as well. In this sense, the Old Testament had always been part of the Christian Bible, and without anachronism, those who believed it were Christians, whether they were Jews or not. The religion of the Jews was not the religion of the Old Testament. Rather, using the background and history provided by it, they built an elaborate system that consisted of their own doctrines and traditions designed to subvert the Old Testament. Jesus explicitly rebuked them for this. Thus it is not outrageous to claim that the Jewish concept of God does not in fact come from the Old Testament (although it borrows from it), and that from Genesis to Revelation, the Bible testifies only of the Triune God.
The Christian God is unique to the Christian faith. No non-Christian shares the same or even a similar concept of God, and if a person does, he is already a Christian.