Attention please：周公恐惧流*言日，王莽谦恭未篡时。(I have checked my《唐诗鉴赏词典》) The answer to the question lies in nowhere but the following line：向使当初身便死，一生真伪复谁知？ The context here is that Bai Juyi's close friend Yuan Zhen had displeased some bigwigs in official circles and was then relegated to a position not so important.In this verse Bai Juyi would like to express his way of looking at what had happened,that is to say,never pass judgements too easily on a person by simply one thing that he had done,otherwise you will confuse the truth and falsehood.Only with the lapse of time the true him/her would finally be seen.So Zhou Gong was never a base person while Wang Mang had never been a man of noble character,whom were put forward as a pair of facts to the arguement in the poem.

白居易..放言五首..之三中诗云 赠君一法决狐疑，不用钻龟与祝蓍。试玉要烧三日满，辨材须待七年期。周公恐惧流言日，王莽谦恭未篡时。向使当初身便死，一生真伪复谁知。 周公恐惧留言日，王莽礼贤下士时 说的是两个人：西周时的姬旦与西汉时的王莽 周公，姓姬，名旦，是周文王幼子，世人惯称“周公”。有圣德，辅其兄武王姬昌伐商，平定天下，定了周朝基业。武王病，周公为册文告天，愿以身相代。藏其册于金匮，内容无人得知。后来武王驾崩，太子成王年幼，周公尽心辅佐，将周成王抱于膝上，朝见诸候。当时其庶兄管叔、蔡叔图谋不轨，但忌惮周公，于是在列国间散布流言，说周公欺侮幼主，图谋篡位。久而久之，周成王起疑。周公为避祸辞了相位，避居东国，心怀恐惧。后来有一日，天降大雨，雷电击开金匮，成王见了册文，方辨明忠奸，诛杀了管叔、蔡叔，迎周公重归相位。 假设当管叔、蔡叔正四处散布流言诬周公有反叛之心的时候，周公便一病而亡；假设金匾之文始终未被成王所知，那么请问，谁人可以说得清周公姬旦到底是忠是奸？那其后的史书中他岂不就成了奸臣？ 王莽，字巨君，是西汉元帝王皇后的弟弟、西汉平帝的舅舅。其人奸诈，依仗外戚专权，阴谋篡取汉家刘姓天下。但他深恐人心不服，于是预先谦虚恭谨、礼贤下士，假行仁义，当时天下人都齐声称颂王莽的圣贤仁义之名。后来王莽终从王太后处逼得传国玉玺，自立为帝。 假设王莽在篡位称帝前就死亡，那谁又会知道他心底的不臣之心？那其后的史书中他岂不就成了一代贤臣，名垂青册了？ 所以说有些真伪，不是一句“日久见人心”或“盖棺定论”就能说得清的，或许身处猜疑之中的人，只能用“无愧于心”来聊以自慰了。