John 5:47

εἰ δὲ τοῖς ἐκείνου γράμμασιν οὐ πιστεύετε, πῶς τοῖς ἐμοῖς ῥήμασιν πιστεύσετε;

And/but if you all do not believe in his writings, how will you all believe in my words


 * οὐ πιστεύετε - you all do not believe. Imperfect active indicative 2P pl.
 * πιστεύετε and the "believe" family of verbs take a dative (believe in), hence τοῖς γράμμασιν are dative
 * πιστεύσετε - Future active indicative 2P pl. With πῶς. How are you going to believe.
 * ῥήμασιν - again, the believe takes a dative, in this case the dative of ῥῆμα. Note the rough breathing. Supposedly we are meant to roll the "r".