Its cathedral is the Holy Trinity Metropolitan Cathedral (Catedrala Mitropolitană Greco-Catolică Sfânta Treime) in Blaj.
During his service the Diocese of Alba Iulia and Făgăraş (centred at Blaj) was removed from the jurisdiction of the Roman Catholic Archdiocese of Esztergom (in Hungary) and became an ecclesiastic province in its own right, with the Dioceses of Oradea Mare, Gherla and Lugoj as suffragans (subordinate dioceses).
In cooperation with his brother Anton Erhard Martinelli, he designed the plans of several baroque churches in the Habsburg empire, among which the church in Grossweikersdorf, the Holy Trinity Cathedral in Blaj and the church in Dunaalmás.