The issue with people who say, "I believe in God.", is that it's often their God they believe in and not the God of all people, of all living things, of this earth, and of the universe. They only believe their God is God despite no religion is a majority in the world.
When they say they believe in God, they are also saying your God isn't God, and to many, even the enemy of their God and them. Doesn't the Bible say God is the God of all men, meaning all people? Doesn't that include people of all religions and faith?
Would God really say what they say?