A branched multiblock polybenzimidazole-benzamide copolymer, specifically, one consisting of a repeating unit represented by Formula 1, and a method for preparing the same; an electrolyte membrane using the branched multiblock copolymer, a consistent electrolyte paste/gel and a method for preparing the same; a membrane-electrode assembly (MEA) using the electrolyte membrane and the consistent electrolyte paste/gel and a method for preparing the same; and a fuel cell prepared from the membrane-electrode assembly. The electrolyte membrane according to the present invention has high hydrogen ion conductivity over a wide temperature range, and excellent physical properties such as mechanical properties, chemical resistance and thermal stability. Deterioration of the membrane properties is effectively controlled by phosphoric acid doping and high hydrogen ion conductivity is realized even with a low phosphoric acid doping level. This consistent electrolyte paste/gel may be useful in improving the operating performance of a fuel cell by coating the electrolyte uniformly.