He is the author of multiple books, including Infiltration: The Plot to Destroy the Church from Within and his bestselling historical fiction series Sword and Serpent.

[3] After studying at Westminster Theological Seminary and Nashotah House, Marshall was ordained a priest of the Episcopal Church in 2005 by Jack Iker, the Bishop of Fort Worth.

[16][17] His 2019 book, Infiltration, claims to demonstrate that, over the last two centuries, the hierarchy of the Catholic Church has been actively subverted to promote Freemasonic power.

[19][20] Marshall, on his YouTube channel, regularly comments on statements from Archbishop Carlo Maria Viganò, former Nuncio to the United States.

[21] In October 2019, Marshall received media attention for publicizing an incident at the Catholic Church's Amazon Synod in which Alexander Tschugguel and an accomplice filmed themselves removing indigenous statues, reportedly of Pachamama, from the Church of Santa Maria in Traspontina in Rome and throwing them into the Tiber river.

[25] Traditionalist Catholic writer Matthew Walther criticized both Marshall's advocacy for then-President Trump and his claims against Pope Francis.