Jesus uses the double negative "Do not be unbelieving" to emphasize the words that follow, "but believe." If your language does not allow double negatives or the reader would not understand that Jesus is emphasizing the words that follow, you can leave these words untranslated. Alternate translation: "This is what is most important for you to do: you must believe" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])