In 1915 Grüber was conscripted as a soldier and fought in World War I until 1918. In early 1918 he started an education as a military chaplain and in mid-1918 he passed the second state examination in theology and was accepted into the preacher seminary (Domkandidatenstift) at Berlin's Supreme Parish and Collegiate Church. He was tutored by court preacher Bruno Doehring. In 1920 he was ordained as pastor in Berlin and took his first ministry in , Dortmund. In June 1923 the French occupation forces prompted Grüber's expulsion from their occupied Ruhr area. Exiled in Berlin, Grüber engaged for the Rhine-Ruhr Relief (Rhein-Ruhr-Hilfe) collecting donations for the poor impoverished by the Occupation of the Rhineland and the Ruhr area. After his return in November 1923 he resumed his pastorate in Brackel, changing in 1925 to the "Düsselthaler Anstalten", a diaconal youth welfare charity of the Inner Mission.