According to Buddhist sūtras, the Buddha’s tongue is as grand as to cover His whole face, as expansive as to cover the triple thousand worlds, and as lengthy as to reach the Summit Heaven in the Region of Form. It is a mark of physical excellence showing that the Buddha has never uttered a false word ever since the eternal past. Therefore, it is preached in a certain sūtra, “There exists no falsehood in the words of the Buddha even if Mt. Sumeru crumbles or if the earth is overturned.” This means that there is never a mistake in the words of the Buddha even if the sun rises in the west or the ebb and flow of the tide in an ocean stop, does it not? Moreover, the superiority of the Lotus Sūtra over various other sūtras has been confirmed by the words of the Buddha of Many Treasures, and the long tongues of other Buddhas touching the Brahma Heaven. There must not be any mistake in the sūtra, not even one written character or one stroke of a character.

Ueno-dono Gohenji, Reply to Lord Ueno, Writings of Nichiren Shōnin, Volume 7, Followers II, Page 4

Coming from a non-Buddhist upbringing, this concept that a long tongue signifies truthfulness fascinates me.