Here is what Rabbi Nathaniel Helfgot, the Chairman of YCT’s Bible Department, who is also the Treasurer of the IRF and one of the featured “Chaverim” on the RCA web site, wrote a year ago:
[Ch]azal and some of the Rishonim, belief in the latter is an article of faith, and denial of it potentially shatters the foundation of the entire structure of the binding nature of Torah. There clearly were Rishonim, such as the Sephardic exegete Rabbi Abraham Ibn Ezra and the Ashkenazic pietistic scholar Rabbi Yehuda HaHasid, who maintained that an isolated section of the Torah was post-Mosaic, a gloss from the pen of a subsequent prophet. However, the notion of the entirely composite makeup of the Torah has no precedent in classical Jewish sources, and it is therefore impossible to term such a theological understanding as Orthodox in any meaningful sense.”
*Mikra and Meaning: Studies in Bible and Its Interpretation*, page 40.
I fail to see how this is heresy.