The Holy Trinity Church was built in the years 1694–1702. Authority of the church and the seminary was the founder Prince Archbishop Johann Ernst von Thun. The church, together with the same time build St. John's Church (hospital church) the first building designed by Johann Bernhard Fischer von Erlach in Salzburg. He used as models various religious buildings in Rome—especially Francesco Borromini's Church Sant'Agnese in Agone in Piazza Navona. While the church is characterized by a discreet use of decorative elements, the entire structure conveys a palatial impression.
The church interior is a longitudinal oval room with four short barrel vaults with crossed arms, which are surmounted by the large drum dome. The monumental effect of the interior arises mainly from the simplicity of the building and the towering height of the dominant drum dome. The design is reminiscent of the inner shape of the Karlskirche in Vienna, which was the second sacral masterpiece by Fischer von Erlach.
The large dome fresco inside the church, Coronation of the Virgin, was executed by Johann Michael Rottmayr between 1697 and 1700. It was Rottmayr's first sacral dome fresco and shows the Coronation of the Virgin by the Holy Trinity with the assistance of the Archangel Michael, as well as other saints, angels, prophets, the ten holy popes, and the Church patriarch. The fresco sums up the Christian salvation and church history impressively together. The figures are grouped in concentrically arranged cloud banks. At the top lantern a Holy Spirit dove is present. The large dome fresco completes the impression of Baroque ecclesia triumphans, the triumphant Church, within the meaning of Baroque self-understanding of the church building.
The stucco work by Andrea Sallari and Johann Baptist Redi is limited in the main room to the capitals, and in the cross arms stucco boxes. The high altar was created in 1700 according to plans by Fischer von Erlach. It was revised in 1841, but was returned to its original state in 1947. The altar contains a sculptural group of the Trinity group with two adoring angels. The reliquary of St. Ernestus was designed in 1959 by Otto Prossinger. The two side altars contain magnificent life-size angels, which were designed by Fischer von Erlach and Michael Bernhard Mandl from 1700–02. The Marie miraculous image of the right side altar dates from the sixteenth century. The Baroque frame was carried out by Sebastian Stumpfegger.
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