The word(s) "believest" appears 2 time(s) in 2 verse(s) in Quran in Muhammad Pickthal translation.

(1) Saying: O our father! We went racing one with another, and left Joseph by our things, and the wolf devoured him, and thou believest not our saying even when we speak the truth.

(سورة يوسف, Yusuf, Chapter #12, Verse #17)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)

(2) His comrade, when he (thus) spake with him, exclaimed: Disbelievest thou in Him Who created thee of dust, then of a drop (of seed), and then fashioned thee a man?

(سورة الكهف, Al-Kahf, Chapter #18, Verse #37)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)