The Katholikon of the Monastery has typical, elegant, Athonite style and its main church is two columned, cruciform, inscribed with the two typical conches of Mount Athos (the choirs). The nave is preceded by an ample esonarthex (lite) with a nice dome in its center, equivalent to the one of the nave, supported by four columns.

The nave, as the inscription on the south wall declares, was decorated in 1548. The name of the painter is not mentioned, yet the style, the colour, the movements, the arrangement of the figures and the scenes and generally the manner of these wall-paintings that look alike with those of the chapel of St Nicholaos in the Monastery of Megisti Lavra (in Mount Athos), which was painted (according to the inscription) in 1560 by the Theban painter F r a n g o s  K a t e l a n o s, lead us to attribute the illustration of the nave of Varlaam Monastery to the same famous artist. On this account, the painting of the katholikon of the Monastery of St. Nikanor in Zavorda of Grevena is attributed to the same artist.

On the dome, there is the representation of Pantokrator (the All-Mighty) as the Fair Judge, in the tympanum there is the honorary chorus of the prophets and angels, on the pendentives the four Evangelists, of whom Luke is depicted to paint an icon of Virgin Mary. On the two lateral conches of the nave, the choirs, there are full-bodied military saints, while on the upper surfaces of the walls there are many-figured compositions inspired by the life and passion of our Lord and the church calendar in general. On the west wall above the entrance is the established representation of the Assumption the center of which is predominated by her lifeless body on the death bed, while Lord is holding gently her pure soul flanked by angels, apostles and hierarchs. Remarkable is the performance of single figures of saints such as of the two melodists of our church, Ioannis Damaskinos and Kosmas Maioumas.

On the east columns of the sides of the iconostasis are depicted full-bodied Virgin Mary on the right and Jesus Christ on the left with richly decorated aureoles over their heads reminiscent of portable icons which happens to the icons of other single figures of saints and archangels (St. John the Baptist, Archangel Michael etc.). On the west columns, below the Virgin Mary, to the left, there is Theophanis and below the Christ, to the right, Nektarios, the two founders of the monastery in monastic frocks looking humble and venerable, offering an exquisite model of their church.

On the sanctuary apse the imposing representation of Platitera (the Virgin Mary) impresses with the glamour of the gold and the rest of its colours as well as with the sweetness in the expression of her face. Below, there is the illustration of the angels as ministers of God full of devoutness.

The marvelous illustration of the nave of the Katholikon of the Varlaam Monastery contains all the characteristics of Frangos Katelanos’ paintings, that is the narrative details and the analysis of the historical events in the compositions on the one hand and the distinct realism on the other, borrowed elements and influences perhaps of the Italian art. Frangos Katelanos couldn’t avoid completely the influence of the Cretan painter Theophanis, yet, as A. Xygopoulos remarked, “his work can be characterized as a reaction to the general admiration and in a way the subjugation to the art of the great Cretan artist.

Eighteen years after the painting of the nave, in 1566, according to official inscriptions, the narthex (lite) of the katholikon was painted by two famous hagiographers from Thebes, the priest and sakellarios of Thebes Georgios together with his brother Frangos at the expenses of the bishop of Vella Ioannina Antonios Apsaras. The two brothers bore the surname Kontaris as it is mentioned by building inscriptions of the churches at the villages Krapsi and Klimatia in Epirus. The church of St. Nicholas in Krapsi was decorated by both brothers in 1563 while the church of the Transfiguration of the Saviour in Klimatia was decorated by Frangos Kontaris alone in 1568.

On the dome predominates the representation of Pantokrator, on the east wall the many figured composition of the Second Coming with all the characteristic scenes of the unbiased judgement, while on the west wall the impressive allegorical painting of the saintly hermit Sisoes, lamenting on the open tomb which has skeleton of the glorious Alexander the Great the conquerer of the world, symbolizes the vanity of the worldly life and the cruel inevitable destiny of every human being without exception which is death: “All the things that the human have, things that do not exist after death are futile. Wealth is not eternal, glory is not permanently a part of man. Thatis, whendeathcomes, allthesedisappear…”. (Πάντα ματαιότης τά ανθρώπινα όσα ουχ υπάρχει μετά θάνατον’ ου παραμένει ο πλούτος, ου συνοδεύει η δοξα’ επελθών γαρ ο θάνατος, πάντα ταύτα εξηφάνισται...)

On the east part of the south wall there is the representation of the two founders Nektarios and Theophanis before their tomps, holding together a model of the church. There is a supernatural peace on their austere and ascetic faces. On the east columns, there is on the left Virgin Mary the Mesitria and Protector of the Christians and on the right Jesus Christ. All around, the walls are illustrated with martyrdoms, full-bodied saints and ascetes etc.

An inscription above the painting of Virgin Mary on the left of the narthex informs us that “in the year of the Lord 1780 and 1782 all the painting of the sanctuary of the Katholikon and the narthex was restored with the help and at the expenses of the modest bishop of Stagoi Parthenios for the salvation of his soul”.

Parthenios was a well-known scholar and energetic bishop (March 1751- +26 March 1784), who had also been a monk of this monastery and great donator and benefactor. Among others, he donated to the monastery his remarkable personal archive and all his library. At his own expenses he built the exonarthex of the katholikon, which was a colonnade with a double row of arches, as it is described in the rhymes of the priest-monk Gabriel Agiamonitis (1786). By the inscription it is known that this narthex remained until 1857 when it was renovated or rebuilt and finally replaced by the present exonarthex with the guest’s quarters on the upper floor, when Polycarpos [Thomas] was the Metropolitan of Trikke and Stagoi and Christophoros Maes the Superior of the monastery, as it is mentioned on the inscribed marble plaque in the year 1930.  

Remarkable for their fine and elaborated art are the woodcarved gilded iconostasis of the nave of the katholikon, the throne of the Superior and two lecterns with rich decoration of ivory; the inscription in one of them informs that they were made during the bishopric of Paisios [Kleinovitis, 1784-1808] of Stagoi and the abbacy of Anatolios.