What do you say to arguments like " Scholars say that the Mahayana scriptures appeared, or were composed, about 400 years later than the Agamas and Nikayas" ??
On what grounds can you consider the Three Turnings to be authentic or inauthentic teaching??
When we know that Dharma existed as an oral tradition for hundreds of years( including the Agama/Nikaya traditions), why do we then accept the so called "dates of the mahayana sutras" without blinking???? http://en.wikipedia.org/wiki/Three_Turnings_of_the_Wheel_of_Dharma
First, I'd suggest going and looking at the very idea of "three turnings".
The formula given in the Samdhinirmocana sutra is not the only one.
Why are there two, at least? What purpose would it serve to have two totally different notions of "three turnings"?
Second, I'd suggest doing some serious study of Buddhist history.
Really serious study. Not just enough to know that such-and-such tradition was oral for some time, but get into the real nitty gritty details of all this.
Third, as for "why do we then accept..." - who is this "we" to which you refer?
I have done very little research in buddhist history, recently I read Hirakawa Akira's book History of Indian Buddhism. On this basis I think that the the buddhist history is told in a wrong way usually. There were many different schools, they were all ORAL TRADITION for 300 years.... up to 1000 years. It is difficult to understand this, it should be explained what an oral tradition is! There still are oral traditions on planet earth, in Africa, Australia, Polynesia etc... This aspect of buddhist history should be explained in detail, it should be emphasized.
The first sutras to be written down were probably Mahayana, atleast they were Mahasanghika sutras. Theravada sutras were begun to be written down in Abhayagiri monastery in SriLanka, quite late, I think. Then after some hundreds of years the Abhayagiri monastery lost in a power struggle and it was wiped out of existence. ( All this is told by Hirakawa Akira in his excellent book)
The sutras of the Mahasanghika, Mahayana and other schools appeared based on the existing stream of oral traditions.