你可以用类比的方法来认识这二个结构的区别。

Tom is a nice person, as you know.

Tom is a nice person, which you know.

如果你能区分这二个句子的差异，你就能理解你的二个句子的差异。唯一不同的是，一个是比较分句，具有比较意义；一个是关系分句，没有比较意义。一个先行项（比较项）为分句；一个先行项（比较项）为名词词组。

Ignorance and superstition，as (were) represented by organised religion，would suffer a mortal blow. as引导比较分句，省略了were.

Ignorance and superstition (which were) represented by organised religion would suffer a mortal blow. 过去分词短语作定语，相当于关系分句作定语。