Lupaș was born in Szelistye, now Săliște, Sibiu County (at the time part of Austria-Hungary). He attended between 1886 and 1891 the primary school in his home village.[2] In 1892 he started attending the State School in Nagyszeben (Sibiu), but, due to a conflict on national topics with his history teacher Árpád Trompa, he was forced to move (together with his colleague Octavian Goga) to the Andrei Șaguna Orthodox School in Brassó (Brașov), from where he graduated in 1900.[2]
He studied Philosophy and Literature at the University of Budapest on a "Gojdu Foundation" scholarship, graduating in 1904,[2] and received his Ph.D. from the University of Berlin (1905) with the thesis The Romanian Orthodox Church in Transylvania and the Communion with Rome in the 18th Century. Between 1905 and 1909, Lupaș taught Church History and Romanian History at the "Andreian" Institute of Theology in Sibiu and attended Theology courses.[2]
During his studies he made his debut in journalism and co-founded the Romanian-language Luceafărul magazine. In November 1907, Lupaș was brought to trial for seditious libel, being accused of having instigated the peasants to hatred against the landowners, and sentenced to three months imprisonment and a 200 Krone-fine.[2] He served his three month-sentence in Szeged between August and October 1908,[2] forced out of the Institute in 1909, and appointed priest to the Săliște parish.[2]
The Romanian Academy elected him an associate member in 1914 and full member in 1916, at Nicolae Iorga's suggestion.[2] However, due to the war, he managed to deliver his acceptance speech before the Academy on 8 June 1920.[2]
Starting with 1919 he became professor at the University of Cluj, teaching Modern History and Transylvanian History until 1946, and in 1920, together with Alexandru Lapedatu, co-founded the National History Institute, located also in Cluj; he also taught Church History at the Theological Academy. Lupaș was elected president of the History Section of ASTRA and, between 1932 and 1935, president of the History Section of the Romanian Academy.