It's pretty much the same as not believing in Zeus, or Pele' or the evil closet-monkey, except those that believe in Zeus or the closet-monkey are much more accepting of non believers than some Christians.
At least they are not trying to make laws to force their beliefs on everyone.
It's like not believing in Santa Claus or the Easter Bunny. Or, more directly, it's like YOU not believing in every version of a divine being in which you choose not to believe. I can only presume you have a specific definition of "God" when you say that you believe, and that versions of gods which don't meet that definition are versions in which you don't believe. I presume you don't believe in the Islamic concept of God called "Allah" or the Zoerastrian version or that of The Church of the Living Elvis or the Flying Spaghetti Monster or Zeus or Jupiter or Vishnu or Brahma or Osiris or Coyote or Quetzalcoatl or the Earth Mother or any of the other multitudes of gods or dieties or creator beings which have been created out of the minds of men and women over the centuries.
You seem perfectly content to not believe in all those versions of God, just like atheists have. You've just made an exception.
That's what it's like.
It confirms my thoughts about people, not God, who have invented several different scriptures, recycling other stories of supernatural power. They are still inventing and believing in new religions even today. As religion only requires faithful followers for it to seem valid and deserving of respect, critical thinking and science must often be considered as enemies of their presumed "truth".
Our need to join in common purpose is largely an evolved social drive that drove groups to conform to any purpose that was beneficial or otherwise pressing for the group's survival. As we quickly became increasingly successful at survival in groups, our need to invent and believe in group purpose had to morph into "higher", intellectual but fictional narratives. Many of these narratives were then ascribed divine origin, easily imposed by force, and used as tools to control culture and behavior, for better and for worse.