The phrase "instant cult" refers to something, like a book or movie, that quickly becomes popular and gains a devoted group of fans who passionately follow and adore it. Full definition
The movie gained a lot of word of mouth publicity and became an underground instant cult classic. (fatmovieguy.com)
That said, it is destined for instant cult status and eventual critical respect. (filmfreakcentral.net)
Instead of the attempts at instant cult films (lookin at you, Sharknado), there are genuine movies that have developed a strong following around them despite not being massive hits upon their release. (avclub.com)