This article considers how ideology motivates lone actor terrorists to use violence. A significant body of literature in terrorism studies sees ideology as a much less important driver of terrorist action than other phenomena such as personal crisis and status seeking. Yet these studies often reach this conclusion without first defining ideology or how it affects behavior. We adopt a broad understanding of ideology as sets of political worldviews which guide thought and action. We use Maynard’s theoretical framework of how ideology can lead to action to assess the way nativist white nationalism influenced Brenton Tarrant to attack two mosques in Christchurch on March 15, 2019, killing 51 people. To do so, we analyze five years of previously undiscovered online posting by Tarrant on 4chan’s “politically incorrect” (/pol/) board. We make no claim to absolute certainty of his authorship, however the presence of a combination of identifiers makes this the most plausible conclusion. We find that ideology was crucial in Tarrant’s pathway to his attack, and influenced his behavior in many of the ways identified in Maynard’s framework.