What are you an atheist about?
Definitionally speaking, someone who believes in at least one theistic deity is a theist. An atheist is someone who lacks belief in any theistic deities.
However, in ordinary conversation, we often limit our discussion to just one or several deities at a time. For instance, if we are just considering belief in the Christian god, Yahweh. Christians are clearly theist when it comes to that god. Muslims however do not believe in the existence of Yahweh, and so with respect to Yahweh, Muslims can be said to be atheists (perhaps even strong atheists).
With respect to Thor, both Christians and Muslims are atheist. They lack belief in the Thor deity. In fact, both Christians and Muslims are atheists with respect to Vishnu, Ra, Thor, Zeus, Venus, the Flying Spaghetti Monster and the thousands of other deity concepts that men have dreamed up. Humanity has an extremely fertile imagination.
Some of these deities can be disproved. It is frequently stated that it is not possible to prove a negative, but this isn't quite true. It is generally not possible to prove a universal negative (e.g. there are no fairies in the universe), but it is sometimes possible to disprove particular concepts. This could be because their described properties are logically impossible or because they are someone bounded in space and time (e.g. I can prove there are no six inch tall blue unicorns in my sock drawer at the moment by opening my sock drawer and looking in it).
Most deities and supernatural entities cannot be conclusively disproved. But it is not a rational stance to assume (and act as though) they are all true until someone conclusively disproves them. You don't find people believing in and worshipping Yahweh and Allah and Brahma and Akka, Vishnu, Baiame, Ra, Thor, Coatlicue, Zeus, Balor, Venus and all the others just because "you can't prove they aren't real".
No, if someone makes a claim about a deity, we don't believe them unless and until we see some sort of evidence. The burden of proof is on the one making the claim. No evidence, no belief. As simple as that.
"We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further." -- Richard Dawkins
"I contend we are both atheists, I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours." -- Stephen F Roberts