Efficacy as a cure for
Dukkha. That is the main test.
I (probably like you, Kojip) have heard any number of historians' and Buddhist teachers' comments on "What the Buddha Taught" ... and the precise content is lost in the fog of time. However, that being said, we can be pretty darn sure of what was at its heart based on how universal certain teachings and themes were in the earliest literature ... the Four Noble Truths/Dukkha, Non-Self, Impermanence, Dependent Origination, some other things too. The central focus of each, in turn, is a radical 'treatment' for the plague of
Dukkha. 'Ol Gautama found a highly effective cure for
Dukkha. (Here is one such scholar I was listening to today):
http://www.audiodharma.org/series/207/talk/2602/
Centuries later, the Mahayana scriptures (even if not literally the Buddha's words) were written by authors infused and inspired by the Buddhist Teachings ... and the writings are nonetheless still focused on these same themes: Four Noble Truths/Dukkha, Non-Self, Impermanence, Dependent Origination, and some other things. The Mahayana writers, in their visions of Emptiness and the like, tinkered with the Buddha's original propositions a little (not all so much really) and found an even more effective cure for
Dukkha. (
I sometimes think of Gautama Buddha as the "founder" ... much like the Wright Brothers for the airplane ... and then Buddhism took off on a couple of thousand years of refinement, change to suit cultures, trial and error, adding and subtracting and evolution until we have a Dharma-747! )
The bottom line is the efficacy in dropping the self, dropping body-mind, and Liberation from
Dukkha. Most of the teachings ... whether an understanding of Dependent Origination, an allowing of Impermanence, living in accord with the Precepts free of hate, anger and ignorance ... are aids in such. The frictions and divisions between our little
self and the
not-the-self life-world drop away.
That is perhaps the One and Only litmus test for what words and teachings are in keeping with the Buddhadharma.
Gassho, J