A fair enough argument, Sanghamitta. However, Christianity entered the picture thousands of years into the history of Judaism and constituted a clear, radical break. By contrast, Mahayana originated only a few hundred years after the Buddha's paranibbana and actually not that long after Theravada itself emerged as a distinctly identifiable school.
At the time when this happened, a variety of schools and practices existed that were all based, or claimed to be based, on the Buddha's teachings. The most credible theory which I've heard concerning the Mahayana is that it developed as a kind of "back to basics" movement among monastics, focused on emulating the Buddha's journey from bodhisatta to buddha, and with a big emphasis on asceticism and seclusion. You can see this, I think, in influential texts such as Zhiyi's "Manual of Samatha, Vipassana and Dhyana Meditation". (http://www.amazon.com/Essentials-Buddhi ... 1935413007
As we were discussing above, "buddha nature" can be connected to "luminous mind" -- though I agree that it did transform into a kind of metaphysical monster. Stephen Batchelor has an interesting talk on this subject -- he suggests a Chinese translation error was instrumental in this transformation.
Maybe the short answer is: for a Theravadin, there may be two different religions. For a Mahayanist, probably not. From an objective scholarly point of view, up for debate depending on how we interpret early developments.