The atheist mocked the bishop's explanation.

Literal

Atheist [subject-が] bishop [possessive-の] explanation [object-を] mocked.

無神論者 = 'atheist' (無 + 神 + 論 + 者, 'one who argues there are no gods'). 司教 = 'bishop' (Catholic clergy title). あざける (嘲る) = 'to mock, to ridicule.' A religious-philosophical confrontation scene.