The theologian Martin Kähler is known for his work "Der sogenannte historische Jesus und der geschichtliche, biblische Christus" (The so-called historical Jesus, and the historic, biblical Christ). He clearly distinguished between "the Jesus of history" and "the Christ of faith". Some historians openly admit bias, which may anyway be unavoidable. Paul Hollenback says he writes about the historical Jesus, "...in order to overthrow, not simply correct, the mistake called Christianity." Another historian who has written about Jesus, Frederick Gaiser, says, "historical investigation is part and parcel of biblical faith."