Olabimtan, Kehinde. "Graf, John Ulrich". Dictionary of African Christian Biography. Dictionary of African Christian Biography. Archived from the original on 15 September 2015. Retrieved 22 February 2016.
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Olabimtan, Kehinde. "Graf, John Ulrich". Dictionary of African Christian Biography. Dictionary of African Christian Biography. Archived from the original on 15 September 2015. Retrieved 22 February 2016.