By that time, Varnava Rosić was serving as a Serbian Orthodox priest in Constantinople and he was chosen and consecrated as bishop on 10 April 1910 in the Patriarchal Church of Saint George.

As an auxiliary bishop serving in the Eparchy of Debar and Veles, he welcomed the liberation of that region from Turkish rule in 1912 and annexation to the Kingdom of Serbia.

During First World War, upon the Bulgarian occupation of southern parts of Kingdom of Serbia in 1915, he had to leave his eparchy, returning after liberation in 1918.

The construction of the edifice of the great Temple of Saint Sava was initiated in Belgrade (one of the largest churches in the world).

Varnava firmly resisted the introduction of legislation giving greater privileges to the Roman Catholic Church not in Yugoslavia in general, but in Serbia in particular (hence The Concordat Crisis).