Did Rengoku Really Say “Set Your Heart Ablaze”?

Yes. Kyojuro Rengoku says "set your heart ablaze" to Tanjiro as he dies at the end of the Mugen Train arc, and the sentence it belongs to is longer and stranger than the three words on the merchandise. The anime titled the episode after it, which is about as verified as a line gets.

The verdict

True. Screen Rant's list of the series' best lines prints the sentence as Rengoku's final words: "No matter how devastated you may be by your own weakness or uselessness, set your heart ablaze." The Mugen Train arc's seventh television episode, the thirty-third of the series, is titled "Set Your Heart Ablaze," and aired in November 2021; the same scene closes the 2020 film. Demon Slayer has no Wikiquote page, so the line rests on the editorial tier, but an episode carrying the sentence as its name is the anime confirming it in the most public way available.

The sentence the lighter leaves out

The instruction is the tail of a speech. Rengoku, mortally wounded by Akaza, tells Tanjiro to live with his head held high, that whatever devastation he feels at his own weakness he should set his heart ablaze and look straight ahead, and then, in the version of the scene most viewers remember, that he believes in him. The poster keeps "set your heart ablaze" because it is a command that works without context. What it drops is the condition: no matter how devastated you may be by your own weakness or uselessness. Rengoku is not telling Tanjiro to be passionate. He is telling a boy who has just watched him lose that losing is not a reason to stop, which is a specific piece of advice from a specific dying man and reads differently on a phone case.

Why it became the series' slogan

Because Rengoku's death is the moment Demon Slayer became the biggest anime franchise in the world. The Mugen Train film was the highest-grossing film of 2020 anywhere, in a year when almost nothing else opened, and the line is its last beat. A sentence that closes the most-watched anime film ever made is going to become a slogan whatever it says; that this one is a good line, and a real one, is a bonus. The episode title sealed it: the television re-cut of the film named its climax after the sentence, and from then on it was the arc's name as much as Rengoku's.

What this site does with it

Demon Slayer's verified lines on this site rest entirely on editorial sourcing, because there is no Wikiquote page to check them against, and each one carries the publication that printed it. Rengoku is not yet on the series hub because that hub requires two sourced lines per character, and this page is the first of his; Tanjiro's page already carries his reaction to the death, the shout at Akaza that Rengoku is the true victor, which is the line this one produces.

How we checked

The verification process asks for a primary or editorial source; an editorial one prints the sentence, and the episode's title confirms it independently. The entry in the fact-check database is true, with the note about the clause the slogan removes.