How can you call it Judaism if it doesn't include God?
God is a part of Judaism, but not all of it. We can believe in the many good values inherent in Judaism without believing in the supernatural authority supposedly responsible for those values. We can feel at one with the history of the Jewish people without believing that God is responsible for that history. We can believe in truth without believing in Revealed Truth. We can have spiritual longings without believing in the necessity of a supernatural being to satisfy those longings.
All throughout history, there have been different beliefs in Judaism, and Judaism may have survived because of an ability to adapt to changing circumstances in a changing world. As a result, many denominations of Judaism now exist. Tradition isn't the only option any more. Humanistic Judaism isn't for everyone, but can reach many Jews who don't accept the traditional approaches. Thus, it's one more way to include Jews who are humanists, and intermarried Jews, who would not be involved with the Jewish community without this alternative. We offer one more way to be Jewish and to celebrate Judaism, for those who want to do this without expressing ideas about a supernatural being or power.