He concluded that the world could be redeemed only by a new religion, whose function it should be to nourish and strengthen the feeble altruism of human.

whole function为定语从句的表语，it 为形式主语，should be为定语从句的系词，to nourish...不定式短语为定语从句的真正主语。

如果删去it，定语从句结构也是正确的，不过whose function就表示表语，而是从句的主语了，不定式也不再是真正主语，而是表语了。

（确认性表语和主语可以互换功能。）