Blink and you miss it: There are several phrases in latin scattered all over the movie. These phrases are shown in at least one frame:
15:54 min 'Abyssus Abyssum Invocat' (Deep calleth unto deep). These words are part of the Psalms, chapter 42, verse 7, of the King James Version.
44:45 'Facilis Descensus Averno' (the downward path to death/Hell Is easy), from Virgil's Aeneid, Book VI, Line 126.
55:07 'Cave Hominem Unius Libri' (Beware the man of one single book). A variant of 'Homo Unius Libri', a phrase attributed to Thomas Aquinas.
1:10:02 'Nihil Pretiosius Veritate' (Nothing is more beautiful than the truth), attributed to Francisco Sánchez de las Brozas, from his opus 'Minerva sive de causis linguae latinae', Book I, Chapter 1.
For Antrum's occult aspects, directors David Amito and Michael Laicini studied various historical and cultural depictions of demons and the devil for inspiration, with their shared interests in religious imagery and the supernatural also factoring into the development of the film's script. Most of the symbols and runes depicted in the film were taken from a 17th Century text called Lesser Key of Solomon.
Ironically, American child actor Rowan Smyth previously starred in the Christian film I Believe (2017) before making this film about a boy who tries to find his way into hell.
A 2018 Canadian mockumentary horror film written and directed by David Amito and Michael Laicini. The film is divided into two parts: an opening and closing frame narrative in the form of a mockumentary and a feature film. The documentary purports to tell the story of Antrum, a movie released in the late 70s that supposedly has deleterious effects on those who watch it; the bulk of the movie is allegedly the only known print of the film, which has itself been altered by an unknown third party.
The makers of the film purposefully did the editing to feature a new and equally different type of sound frequency that is rumored to have a negative effect on your nervous system.