In 1858, when Edo bakufu (Japanese feudal government headed by a shogun) requested for imperial sanction on The Treaty of Amity and Commerce between the United States and Japan, Tadahiko submitted a written opinion to Imperial Prince Arisugawanomiya Taruhito, Dazai no sochi (Governor-General of the Dazai-fu offices) and the eldest son of Imperial Prince Takahito, saying that 'foreign countries are publicly seeking trade but are privately trying to propagate heresy (referring to Christianity).'
安政5年（1858年）、日米修好通商条約の勅許を江戸幕府が要請した際に、忠彦は幟仁親王の長男・大宰帥有栖川宮熾仁親王に対して「諸外国は陽に貿易を求めて陰に邪教（キリスト教の意）を布教せんとしている」という意見書を出した。