The Trinitarian Church, somewhat underestimated by scholars, unjustly remains in the shadow of other Baroque buildings in Vilnius. Its octagonal ground plan, with a lofty "dome" vault of an unprecedented span, places it among the leading central churches of the Baroque era in the Polish-Lithuanian Commonwealth.

The Order of the Holy Trinity, formerly the Order of the Holy Trinity for the Redemption of Slaves, commonly known as the Trinitaries, was founded in 1193 to free, or rather ransom, Christian slaves from the hands of the Muslims during the Crusades. In the modern era, it was also involved in ransoming captives from the Turks and Tartars. It was brought to Poland in 1685 and during the First Republic 30 Trinitarian outposts were established. The first in Lviv, the second in Warsaw. The order was brought to Vilnius by Casimir Jan Sapieha, Grand Hetman of Lithuania, and his foundation was the sixth in order, but the first in the Grand Duchy of Lithuania. The temple was built between 1694 - the laying of the foundation stone and the foundations - and 1716. At the turn of the century, stucco work was done, in 1712 the dome was covered with sheet metal, and in 1716 it was consecrated and put into use. The finishing work continued for some time, and the consecration did not take place until 12 September 1756. The designer of the church is unknown. The stucco decoration of the nave, the church façade and the façade of the monastery is the work of Giovanni Pietro Perti, creator of the earlier exquisite stuccowork of the nearby Church of St Peter and St Paul in Antokol. The church was given - at the founder's emphatic request - a second name, that of the Most Holy Saviour, in addition to that of the Holy Trinity, which is in accordance with the order's rule. It should be noted that the church is not oriented, but faces north with the altar section, has a south-facing façade and is adjoined by a monastery to the west. This is important information for further consideration.

The plan of the temple, for the time of its construction, is innovative; it is the second church in the 17th century Republic to have a regular octagonal plan. The earlier Philippine church in Swieta Gora near Gostyn (1676-1698) in Greater Poland, which was a slightly modified copy of the church of Santa Maria della Salute in Venice, has a span of 17.20 m on the perpendicular axes and 18 m on the diagonal, while the Vilnius church has a span of 18.20 m on the perpendicular axes and 19.60 m on the diagonal of the octagon. At the time, it was the largest such vault in the Republic. The span of its vaulting posed a significant challenge to the builders and was unattainable for local contractors.

The plan of the Trinitarian church is characterised by a rare "conciseness", compactness. Altar niches are cut into the thick walls of the octagonal nave - no spatial element of the nave extends beyond its outline. It should be noted that it has no separate chancel; the main altar is set in the space of the nave. This is unusual, especially for a monastic temple. The centralising interior clearly indicates the function of the mausoleum, or more precisely: the family mausoleum. It is paradoxical that the founder of this work was not laid to rest in its vast vaults after his death in 1720.

The octagonal nave is covered by a lofty vault, which - contrary to frequent descriptions - is not a proper dome, as such a dome should be founded on a circular plan. The vaulting of the Trinitarian church consists of eight Dominican vaults and let us stay on this statement without going into the intricacies of the definition. By comparison, the canopy of the 'dome' on Florence's Cathedral of Santa Maria del Fiore also has this construction. The designer of the Vilnius church, in order to strengthen the stability of such a large structure, following the example of builders of medieval Italian baptisteries or even the Florentine cathedral, sharpened the profile of the vault, giving it an almost pointed cross-section. This sharpened shape distributes compression and expansion forces in a completely different way, similar to arches and Gothic vaults. In the Trinitarian church, this affected the proportions of the interior space. The nave to the top of the beam is about 11.30 m, while the vault from the beam to the base of the lantern is about 15.70 m. Clearly the octagon of the vaulting dominates here. The structure of the vaulting raises an important question. Well, according to Piotr Bohdziewicz's 1943 measurements, its shell has a large and variable thickness of about 1.50 m at the base, about 1.30 m above the niches of the large windows in the dome, and about 0.65 m at the top of the dome. It would be structurally difficult to 'lose' the thickness of a brick wall in such a way as to smoothly reduce the cross-section of the shell, i.e. to go from a weft of five bricks to a thickness of two bricks. Historic domes or vaults had noticeably thinner shells, and could not be too heavy so as not to strain their own structure and the supporting walls or pillars. Typically, in modern architecture, the thickness of the shell of a vault or dome with a span of up to 20 m is a thickness of one brick, sometimes 1.5 bricks and - probably very rarely - a thickness of two bricks. In short, if the Trinitarian vault had a uniform structure with a varying thickness of 1.50 to 0.65 m, then its mass would represent an extraordinary load on the vault itself and also on the supporting walls and thus a significant threat to the stability of the entire nave.

Therefore, it cannot be ruled out that its creator used a structural solution such as exists in the 'dome' vault of Santa Maria del Fiore in Florence. It is about 42 m in diameter and consists of eight dominican vaults with a 2-layer structure. The inner vault ranges from 2.42 m, to 2.10 m thick, the outer vault from 0.75 m to 0.58 m, and the distance between them, or the air cushion, is 1.23 m - in total, at the base, the whole structure is 4.40 m thick. Brunelleschi thus gave the inner shell walls an unprecedented thickness. He stabilised the structure by introducing a truss of masonry vertical ribs and horizontal transoms between the two shells. I am aware of two more repetitions of two-tiered domes in Baroque Italian architecture. The first is a regular dome on a circular plan with a diameter of 41.70 m - in St Peter's Basilica at the Vatican. Designed by Michelangelo, it was only erected with adjustments to the cross-section and thickness of the two layers in 1588-1590 by Giacomo Della Porta, and he is the actual creator of this enormous structure. The other undoubted reference to Brunelschi's 'dome' is the octagonal vaulting of the mighty Capella dei Principi, the Medici tomb chapel at the Church of San Lorenzo in Florence, designed by Mattea Nigetti. Started in 1604, it has a two-tiered dome erected between 1625 and 1656 with a diagonal of 29.7 metres. What makes the Vilnius church different from the cited Italian examples is the absence of a tambour. But the body of the nave should be regarded as the tambour here.

Such double-layered, thick solutions were unique. Is the Trinitarian thick vault also two-layered, and if so, how was it built? I believe that the load-bearing function here is fulfilled by the gurtzes at the junction of the vaults, linking the two vault shells together with a solid wall and bound by a wide ring under the lantern. A cushion of air remains between the staves. This thesis needs to be confirmed either by thermal imaging camera tests or by controlled simple drilling of the vault.

The two façade towers standing against the diagonal walls of the octagon are the second original solution in this church, unprecedented at the time. From the beginning of the 18th century, i.e. later than the Vilnius church, several churches with diagonally aligned facade towers were built in the architecture of the Habsburg state. However, they enclose the body of the temple with a longitudinal, not central plan, and this is a different solution. The Trinitarian towers were built at the same time as the corpus, as evidenced, among other things, by the rear elevations of the second storey of the towers with full pilaster articulation visible in the attic of the front section (rebuilt shortly after the church was completed). The original perceived effect of such a dynamically designed façade is nullified by the cubic porch added in 1802.

The only source-recorded artist associated with the Trinitarian church is the stucco artist Giovanni Pietro Perti (died in Vilnius in 1714). He is an artist known for his earlier excellent work in Vilnius, for his extensive decoration of the nearby Church of Saints Peter and Paul in Antokol and for the Casimir Chapel at the Cathedral. He was a master of shaping human figures in moving poses and in plastically flowing robes. In the local church he designed and executed the stucco decoration of the façade, also the façade of the monastery, but above all its interior, between 1700 and 1705. Unfortunately, in the 19th century, the large, almost full-figured figures of the twelve apostles in the lintels of the nave's arcade niches were destroyed. Fortunately, quite a large group of angels remained. They can be seen above all in the frieze of the beam, where they support the 'Arma Christi'. - is a reference to the church's call. In addition, they have peopled the window cornices in the vaulting, and small angels lean out of the clouds surrounding the base of the lanterns. The decoration of the arcaded niches came from the hand of a completely different, anonymous stucco artist and represents a distinctly lower level.

The regular centrality, without a chancel, the lofty "dome" vaulting, point to two ideological aspects of the temple. The first is related to the additional call of the church - the Most Holy Saviour. Churches dedicated to Christ since early Christianity, since the Jerusalem Anastasis, often had a central plan. However, the second dominant aspect was that a proud magnate aspiring to the royal crown erected what was then the largest 'domed' church in the Commonwealth as a family necropolis.

In the modern Republic, the archetype of the ancestral mausoleum is the royal Sigismund Chapel on Wawel Hill, which was imitated mainly in ideological terms by the erection of domed chapels. Ancestral necropolises were also built on a cruciform plan, but also of a different form. In the Grand Duchy of Lithuania there are two earlier monumental churches serving as mausoleums. The first is the Camaldolese church in Pažaislis, founded by Kristupas Žygimantas Pacas in 1664. The large hexagonal church with a domed dome and a two-tower façade, with a multi-faceted programme of ideas, was mainly the family necropolis of the chancellor. The second is the Church of St Peter and St Paul in Antokol in Vilnius (1668-1775), a monument to the Hetman's pride and the burial place of Michał Kazimierz Pac, a relative of the Pažaislis founder. It has a traditional cruciform plan with a tambour dome on the transept and a two-tower façade. However, the two Pacs' foundations, different in terms of architecture, share a common idea - that of a family tomb.

Sapieha's foundation - apart from being a pious foundation aimed at rescuing captives from Turkish hands - was also a family necropolis. The church was built in the immediate vicinity of the magnate's monumental palace and was linked compositionally to his residence. It is not oriented - although in the open space of Antokol it could have been - but faces south towards the palace park and is set on the axis of the transverse avenue of this garden, from which a monumental gate, a private passage to the church, originally led. The positioning with the two-tower façade facing south had another aspect. In this way, Sapieha's foundation faced the city, as did the nearby church of St Peter and St Paul. The two towers were undoubtedly an attempt at dialogue with the rival - in every respect - church of the foundation of Michał Kazimierz Pac, after whom Sapieha took over not only the great Lithuanian mace, but also the ambition to create his own after the dominance of the Pac faction. It was thus a political demonstration in relation to the rival family. The octagonal church with towers is also an ideological - I emphasise ideological, because not architectural - reference to another mausoleum, the Pažaislis of Krzysztof Sigismund Pac. It is a paradox that neither Sapieha nor almost anyone from his family is laid to rest in the vaults of this temple. His entire family lies separately. Kazimierz Jan Sapieha himself was laid to rest in Bereza, in the Carthusian church of St Joseph (1648-1664), which had been the Sapieha mausoleum since the death of its founder, Lithuanian sub-chancellor Kazimierz Lew Sapieha (1609-1656). By 1747, fourteen representatives of the family were buried in its vaults. Nevertheless, a monumental central church was built in Antokol, which had little in common with Trinitarian monastic construction and local architectural tradition. The plan of the temple, the huge vault, the innovative connection of the towers to the octagonal body - are not the result of the founder's whim, but of the designer's invention. I believe that this solution may have been inspired by the huge octagonal Capella dei Principi Medici in Florence, well known to the church's designer.

The creator of the Trinitarian church is unknown. However, a suggestion of the designer has appeared in the literature, thanks to an unpublished dissertation by Wojciech Boberski. It was Giovanni Battista Frediani - a colonel in the foreign troops of the Grand Duchy of Lithuania, a military engineer (not just an architect) from Lucca in Tuscany. Frediani became famous as the builder of a wooden single-span bridge across the Neris, destroyed by an ice floe. The design of this bridge was to be innovative and unique. He was an engineering consultant to Pažaislis and Antokol to Michal Kazimierz Pac. In 1670, he settled in Vilnius and was first in the service of Michał Kazimierz Pac, and after his death, as it were, 'inherited' by another Grand Hetman of Lithuania - Kazimierz Michał Sapieha, to whom he designed the palace. Military engineers were held in higher esteem than architects and builders in the modern era, their knowledge being much broader than that of a 'civilian' architect. As confirmation of these opinions, it is worth mentioning that the correct elliptical dome in plan (measuring 16 x 21.5 m) on the Dominican Church of the Holy Spirit in Lviv was erected in 1745-1759 by the military engineer Jan de Witte. I believe that Frediani, titled an engineer and therefore having the knowledge needed to build fortifications, was the only builder in Vilnius who was able to erect the vault of the Trinitarian church.