Don't make me ask the Muslims to pray at you...
Perhaps some day you will realize the only true God is Allah
"Allah" simply means "God."
Anybody can believe that God exists. It is evident from nature, and science has even proven it.
The questions about God don't have to do with the fact that many people believe in Him. The questions are about His attributes and promises to people.
The most important promise of God is that He is offering salvation for Heaven, to people who believe in His salvation offer. The difference between Islam and Christianity regarding this offer of salvation has to do with the circumstances.
A Muslim's faith teaches him that his salvation is based on the balance between the number of good works that he does, and the number of bad works. Islam doesn't explain the reason why God would allow a person to have bad works at all, and still get into Heaven.
The Christian religion explains that Jesus, the Son of God, was strong enough (because He is God) that He could take the punishment in Hell for all the wrong things that people do. Then, out of love, Jesus offers forgiveness that He has earned through the punishment, so that people can be saved even though they are evil. The punishment has taken the evil away, but salvation must only be given to those who accept the offer through Jesus salvation. Why? Because there is no other salvation.
Perhaps Islam will come out with a way that a perfect God will allow imperfection to get into Heaven. So far, Islam hasn't explained this. Once they do, people might have a reason for basing their faith on Islam. The only thing left then will be to find out which salvation method is truth.