Complex sentence: The authorship of the Epistle to the Hebrews is uncertain; most modern scholars generally agree that it was "not" written by Paul, and since the third century AD doubts have been raised about the identity of the true author.
Split into simpler sentences: The authorship of the Epistle to the Hebrews is uncertain. Since the third century A.D. doubts have been raised about the identity of the true author.