How can we belong to a church which does not accept Bahá'u'lláh and His message?

As regards the question you asked him: he feels that the Bahá'ís in Germany should, by all means, follow the same procedure as regards withdrawing from church membership as that in America. You should point out to the believers that, by belonging to other organized religious bodies, we are not acting openly because we firmly believe Christ has come again--so how can we belong to a church which does not accept Bahá'u'lláh and His message as the fulfilment of Jesus' message and the reappearance of Jesus Himself?

http://bahai-library.com/writings/shoghieffendi/ldg/vol1/sec-66.html