I beg to differ. Some atheists so indeed believe in things that are unprovable and untestable.
For instance, it is not impossible for something or someone to exist that we are unaware of or do not have the means to test. Being a stereographer by profession, I must often work with very complex formulas to manipulate light and the illusion of depth for digital motion picture projection. The theory of relativity--something I thought would never be a daily variable in my work--strangely enough comes into play for me daily.
Because of a phenomenon created by the way light travels and stops due to polarization, an image which in reality has one series of dimensions in real life will be seen with the same dimensions invert during playback. Such a problem was once thought impossible according to science, but was discovered during the processing of one of the first digital 3D films called Meet the Robinsons.
To make a long story short, artists were baffled when they disovered that an image on a table that was logistically unimportant to a scene suddenly became inverted as a characted walked and stood behind it. Once the character moved again, the inverted effect stopped.
There was no explanation for this for a while. We scrambled to find an explanation while the artists at Disney reconstructed the scene entirely. During the processing of several later films, the effect came about again. Scenes had to be reblocked and reshot to avoid a phenomenon that, scientifically speaking had no explanation and was thus impossible, even though we could now witness it and even explain and partially predict it.
Except for me and one other person, a group of stereographers who studied the process to "redefine" the science behind the problem assembled that was composed of atheists. For one year we could do nothing but demonstrate and off-and-on predict the inversion effect which violated the laws of light. What we were witnessing was completely impossible. When the head of the study tried to get help from several universities, we were astonished when our findings were called "hogwash," even being told that "it would be easier to prove the existence of unicorns or God" by one professor.
Eventually a connection was found between the inversion "illusion" and the theory of relativity. We totally understand the process, what causes it, and how to prevent it from happening. But the whole scientific explanation opens a can of worms for it suggests that reality is not absolute but relative. Our perseption of reality is not always what it seems, and even objective physical evidence can, under the right circumstance, fool the senses into believing that what is, well to put it bluntly, just isn't.
This means that our perception of what is real is always subject to being false. The belief of one or even a million people based on scientific evidence can be proven false because reality is subject to its own objectivity of being relative. Light, time, and gravity can change everything you now accept with the simple change of a minute variable.
In other words, it can now be proven that reality itself is subject to change and is not the objective means of measuring what is and what isn't.
In light of this, an atheist can indeed believe in things that are not true. In fact, once we discovered the way to explain the inversion problem, we laughed at ourselves. Why?
Because the depth we witness via the new 3D process is both illusion and reality. We know this as fact. It is witnessing an illusion that is reality. Depth is real even though though in 3D projection this reality is subjective. The answer we were searching for is based on the same principle. We had to stop thinking of illusions as "unreal". Once we did that, we solved the problem.
And each time you go to see one of these movies in 3D, I guarantee you, you are witnessing reality that does and does not exist at the same time. It is more than illusion and it is only illusion at the same time. And it happens regardless if you believe in 3D or not.
So you can't argue that atheists don't believe in the unprovable and untestable. Too much exists that falls under that catagory. If you don't believe me, study the debte over relative vs. absolute reality.