The most universal Trinitarian symbol is the Trinitarian cross, represented in thousands of places around the world and easily associated with the redemptive charism of the Order of the Most Holy Trinity.

Throughout history it has had multiple representations but always maintaining the unity of the two colors that make it characteristic: red and blue, confessing the faith of the Church in the true divinity and true humanity, respectively, of Christ.

We begin our visit to the church of San Carlino in front of the facade. All of it invites us to travel the road with Mary. The crown of 12 stars that Borromini places on our heads as we enter identifies us with that portentous sign in the sky; it is the pilgrim people ascending to meet their Lord.

With our imagination, let us try to go back in time, before pollution destroyed the picture in the upper oval, where the Holy Trinity crowned the Virgin Mary; she represents the people of God ascending to the bosom of God the Father; she represents us.

The Trinitarian Fathers had proposed the typical facade of the Spanish barefoot churches, which Borromini analyzed and reinterpreted. The Castilian facade revolved around the Trinity, three doors giving access to the Church (Father, Son and Holy Spirit) and the all-seeing eye culminating it.

But in Borromini's fac these three doors will evoke an itinerary: the way of the cross of San Juan de Mata.

If we look at the two external windows, we see a cross on the horns of a deer: it is the cross seen by St. John of Mata and St. Felix, founders of the Trinitarian Order; it is the spiritual path that is offered to us, from which are born two palms of martyrdom, sign of the offering of the life of the saint, of his testimony and of his reward, which will be crowned in heaven. This is the door, the way, the destination.

Continuing with the same concept, the niches of the saints are placed above the doors: St. Charles Borromeo, the titular saint of the Church, is shown adoring the Holy Trinity in expectation of the encounter; the two founders offer us their work as a way to reach that goal.

In the upper part, the niches are not occupied by anyone, they are open doors that invite us to enter another dimension; the central window that in the Spanish churches gave light, here is a balcony that opens to the world. Above them, cherubs carry the cross, surrounded by palms of victory, which also mark the cornice of the balcony; they are the crowns of victory of the saints who wish to enter through here.

Finally, at the top we find the goal, the image of the Trinity, which in the Spanish facades was barely a triangle with a bull's eye, and which here is transformed into a window to heaven, where Mary, our example, already enjoys the crown of victory that her Son has conquered.

In drawing the ground plan of the church of San Carlino, Borromini was inspired by a church founded on the cross and dedicated to the Holy Trinity, which he hoped to find.

According to the rule of life of the Trinitarian Order, the horizontal axis should be very short, giving the width of the church, and based on that width, the rest of the temple should be projected proportionally; thus, the proportion of a church consecrated to the Trinity is the equilateral triangle.

Borromini evoked the measurements of St. Peter's Basilica in the Vatican to highlight the link of both temples with the cross.

But also the dimensions of the church of St. Carlino are related to the Church with a capital letter, the New Jerusalem, which serves as a guideline for the plan and the elevation in the measurements of the Apocalypse. The rule of Scripture, the rule of the Order and the rule of the Petrine magisterium lead us through the cross to the presence of God the Trinity.

The image of the New Jerusalem-"On the east three gates, on the north three gates, on the south three gates, on the west three gates" (Rev 21:13)-which is repeated in the crypt, literally folds the sacred space, making us see that only within the Church are these gates open to us.

The spaces to which these three doors open are also significant:
- To the East, the chapel of the Incarnation, with a repose in Egypt where the angel shows the cross to Jesus as the beginning of his human journey and of our redemption. From here also comes the only light that illuminates the nave of the temple.
- To the west, diagonally, the Chapel of the Cross, with images of the Passion.
- On the other axis, almost making an architectural parallel to the pictorial motifs, the staircase that ascends and the door that communicates with the cloister.

The main altar has the same elements as the facade, to which it adds a Eucharistic meaning: the three doors are placed vertically, in the double tabernacle and expositor, while the three saints and the Holy Trinity are placed in the fresco.

The first idea is to present the cross as the guide of St. John of Mata's journey from Cerfroid, France, where the Order originated, to Rome; there, Pope Innocent III will approve his project of redemption and recognize him as inspired by God the Trinity, giving him the church dedicated to the apostle St. Thomas in Formis, located on the hill of Celio, and the adjoining monastery. St. Thomas in Formis is the initial stage of our journey.

Borromini wanted to understand the spirit of the Trinitarian religious to be able to realize his work, and for this he went to Santo Tommaso in Formis, assuming these concepts and formulating from there the facade of the new convent and probably the central axis of the project.

Thus, the red and blue cross and the Redeemer in the attitude of exchanging two captives inspire the semiotics of the whole, on the cross and redemption.

The path taken by St. John of Mata towards Rome, under the sign of the cross, will rule the horizontal axis at the height of the pendentives of St. Carlino; while the ascent of the same cross from the pavement of the temple to the dome represents the mystery of the redemption on its vertical axis, from the altar and the tabernacle to heaven.

Borromini depicts in the ovals of the pendentives this Roman journey of St. John of Mata from his meeting with St. Felix of Valois in Cerfroid to Rome and the approval of the Order.

The path of St. John of Mata shows us the cross that is presented as a liberating vocation through the Church and the Pope for the good of the captives. A task that identifies us with the work of God the Trinity, of the Father who sends the Son to rescue us from evil; this will be the image that will be reflected in the structure of the temple.

There is another apocalyptic reference that has to do with the location of the temple, but we cannot be certain that it was known to Borromini.

The place where St. Carlinus is located was called in Roman times "ad malum punicum", literally 'Phoenician apple', that is "pomegranate tree". The pomegranate as a symbol of the Passion will use it in the capitals that frame the altars, together with the rose; while the capitals that support the pendentives are crowned with laurel and palm, symbolizing glory.

The cross is a sign of the victory of Jesus, and for this reason it is crowned with glory, with laurel and gold. This symbol will reach its culmination with the double crown of gold and laurel that crowns the nave and divides the pilgrim Church from the one that ascends to the glory of heaven, adorned with its jewels.