Donald J. Trump's father was indeed called Frederick Christ Trump.

The explanation is simple. Donald Trump's paternal grandmother, Elizabeth Christ Trump (10 October 1880 – 6 June 1966), was born in Bavaria as Elisabeth Christ. Christ is quite a common surname (family name) in Bavaria.

In 1902 Elisabeth married a Friedrich Trump (14 March 1869 – 30 May 1918). Friedrich and Elisabeth moved to New York and at some point became Frederick and Elizabeth. They set up house in an apartment in the predominantly German quarter of Morrisania in the Bronx. That was where their son, Frederick Christ Trump, was born on 11 October 1905.

Frederick Christ Trump died on 25 June 1999 in New York.

More Info: en.wikipedia.org