But how can you blame them? They're taught to believe in their God and told what to believe ever since they were a child. It's very hard to change someone's views when they've been told the same thing their whole life.
Sure, it may be true for some, but it is not true for all people;
some people come to believe in ONE God through their own efforts at self-education.
I am an example of one who formerly did not believe, but sought the truth and did find it;
it is not all that unusual, my friend; not everyone who believes is brainwashed from birth.
There are even famous atheists who change their way of thinking; one example is:
The problem with religion is that it attempts to separate the human from God;
God resides within you, everyone is capable of following the golden rule, it is ingrained into human consciousness.
All monotheists acknowledge only ONE God, so they are really all talking about the same intelligence as described by Anthony Flew above.
What made you believe in your God and not some other God? In the end, don't they all practically give the same message?