My last post expressed my belief that Islam and Christianity do not believe the same things about God (and in that sense do not believe in the same God). The central feature of Christian theology is the role and person of Jesus Christ. Something strange is at work in the Christian worldview. We begin with the mysterious claim that the eternal God became a human being. The Old Testament hints that such a thing is not as unthinkable as it seems.
God comes to us. He speaks a word to us. This communication is housed in human language. That means "the Eternal" is clothed in "the limited, the temporal." We call it revelation. Some things are revealed to us in creation. This "natural theology" refers to what we can discern through our intellect. This is what Paul refers to in Romans 1:19 (God can be seen through the things He made). Revelation is what God has shared with us and revealed to us, generally through some human witness (Moses, prophets, biblical authors, perfectly in Jesus). The Bible is called the Word of God. There is also a long chain of teaching and practice which conveys to us the identity and will of God. This is called Tradition. Reason and revelation (in bible and tradition) are the basis of what we believe about God.
So if there is a God why are there so many religions and so much disagreement? I think many people assume that if God exists then everyone should "see" Him and believe the "same thing." It is not clear to me that that is true. I think this is an important point to reflect on. If God wants human beings to be free then isn't it necessary that He give them space? From the beginning it seems that God was both absent and present. Read about Adam and Eve in Genesis 2&3. God showed up to walk with them in the evening when it cooled down. They make the decision to eat the fruit and hide from God. Even before the sin there was a time of absence and a distance. Even before the sin people were capable of making independent decisions. I think we need to remember that absence and distance are part of God's relationship with us. From the beginning.