On the Church Windows
The clear windows that exclude weather and let in the light are the doctors who stand against the storms of heresy and shed the light of the Church’s doctrine upon us. Light shines through the glass in the windows, and this glass is the mind of the doctors, which contemplates the heavenly things hidden the figures as if in a mirror.
On the Columns
The columns that hold up the house are the bishops, who by their righteousness raise the frame of ecclesial life to lofty heights. The beams that join the house together are the princes of the world, who provide protection and support to the Church. The shingles of the roof, which repel rain from the house, are the soldiers who protect the Church from pagans and enemies.
The painted panel-work represents the examples of the just and the beauty of virtue for all in the Church. Now painting is done for three reasons: first, because it is the literature of the laity (laicorum litteratura); second, to give the house fitting decoration; third, to recall the lives of our forebears.
On the Halo
The lights that are depicted around the heads of the saints in the church in the form of a circle signify that they are crowned in the light of eternal splendor. Now it is painted in the form of a round shield because they are defended by a divine protection as if by a shield. Hence they gratefully sing: Domine ut scuto nos bonae voluntatis tuae coronasti (Psalm 5). The practice of sculpting images is taken from the Law, where Moses, by God’s command, made two cherubim out of gold (III Kings 6). But the practice of painting churches takes its beginning from Salomon, who placed various engravings in the temple of God. The use of the candelabra and thurible began in the Law.
On the Pavement
The pavement that is trod underfoot is the common man by whose work the Church is sustained. The crypts built under the earth are those who lead a more secret life. The altar on which we sacrifice is Christ on whose merit the Church’s sacrifice is accepted. The body of Christ is confected upon the altar, because the people who believe in him are refreshed by it. They become one with Christ, just as many stones are made into one altar. Relics are hidden in the altar because in Christ are hidden all the treasures of wisdom and understanding (Colossians 2). Pyxes [containing saints’ relics] are put on the altar; they are the apostles and martyrs who suffered for Christ. The linens and other cloths that decorate the altar are the confessors and virgins, by whose works Christ is adorned .
 The Pontificale provides a similar allegorical interpretation, in the bishop’s admonition to candidates for the subdiaconate:
Cujus altaris pallae et corporalia sunt membra Christi, scilicet fideles Dei, quibus Dominus, quasi vestimentis pretiosis circumdatur, ut ait Psalmista: Dominus regnavit, decorem indutus est. Beatus quoque Joannes in Apocalypsi vidit Filium hominis praecinctum zona aurea, id est, sanctorum caterva.
On the Cross
A Cross is set up above the altar for three reasons. Firstly, just as the insignia of a king are displayed in a royal city, so in the same manner our Cross, the banner of our King is hung the house of God, so that his soldiers may adore it. Secondly, so that the Church may always have a representation of Christ’s Passion. Thirdly, so that the Christian people may imitate Christ by crucifying their flesh to vice and concupiscence (Galatians 5). The vexilla are set up for this reason, that the Church may always be mindful of Christ’s trophy.
On the Propitiatorium
The propitiatorium over the altar is the divinity of Christ, which intercedes (propitiatur) for the human race. The stairs, by which one ascends the altar, are the virtues, by which one strives toward Christ . The lavacrum by the altar where the ablutions of hands take place, is the mercy flowing from Christ, which which men in Baptism or in Penance are cleansed from their filth .
 Ibunt de virtute in virtutem.
 It was once common to have an ablution basin in the wall on the epistle side of the altar. They can still be seen in older churches.