Complex sentence: Later, Lincoln teamed up with two other authors, and co-wrote the 1982 book "Holy Blood Holy Grail", in which unaware they were relying on forged documents as a source, they stated as a "fact" that the Priory of Sion had existed.
Split into simpler sentences: Later, Lincoln teamed up with two other authors, and co-wrote the 1982 book "Holy Blood Holy Grail". Unaware they were relying on forged documents as a source, they stated as a "fact" that the Priory of Sion had existed.