@moonbus saidIt believes in the transmigration of... something.
Buddhism is a form of spirituality which does not require belief in, or presuppose the existence of, supernatural entities.
You could make the argument that old Buddhism was hard atheist, but it doesn't really make sense in the context of how modern Buddhists throughout the world tend to believe, and how we can imagine the average Buddhist generally believing throughout the ages, and the tight little web of interpretations necessary to maintan that it is possible to be atheistic is always quite an exercise...
Unless they assert that it simply rejects godhead, but does acknowledge immaterial realities that allow for the transmigration of consciousness and karmic functions in some real, comprehensive fashion. Even then, it detaches Buddhism further from the way that it tends to be understood and practiced even by very modern Buddhists like Thich Nhat Hanh.