The Forbidden Lore (Cults) skill allows knowledge of the 'most notorious Imperial cults'. How does this differ from Common Lore (Imperial Creed), which basically is about THE imperial cult, and for example Common Lore (Mechanicus) which details one of the most famous imperial cults, namely that of the tech-priests.
I find the various lore skills so vaguely defined and overlapping that it's hard to see what covers what.