This is another myth of our age. The belief in God is too generic to form a common basis. The Christian God is not the Muslim God, and I'm referring to the concepts of God.
To say that all religions are the same (they all involve a belief in God) is equivalent to saying that all physical theories are the same (they all deal with time and space).
A concept takes its meaning from the theory of which it is part.
So, for example, Newtonian time and space are different from Einsteinian time and space. When a new physical theory (relativity) is developed, it redefines the concepts of the old theory (classical mechanics) in a way which may completely transform and revolutionize them.
So is the same for the concept of God in different religious doctrines.