Although it was over forty years ago, I still remember vividly a morning eucharist in the church of the Community of the Resurrection at Mirfield in Yorkshire, where I was studying for ordination to the priesthood. The preacher was my teacher and spiritual director whom I greatly respected. Fr Cedma was elderly, frail and retiring, and only occasionally preached to the community. It was the Sunday before Advent, the last Sunday of the Church's year. His text was from the prophet Jeremiah, looking back over the past to the forgiveness of God and forward in hope to God's future. At that moment he hesitated, then slipped sideways to the floor and lay motionless. We sensed that this was final, and he had indeed died, immediately, from a heart attack. There was an intense silence and in that silence the sun emerged from the Yorkshire clouds and flooded the church with light. For those of us there, it was the light of God's presence transforming that moment.

This book contains twelve studies which explore different ways in which natural light shines in churches to make God present to the worshipper and so enable an encounter with the divine.

The papers collected here were presented at a workshop held in 2020 in Berlin. Each essay describes and presents the example of a medieval church building – or group of buildings – and shows how those who planned and carried out the design used the natural light of the sun to evoke a sense of the presence of God. There are over a hundred illustrations and these, alongside the text, give a vivid picture of the church and the different effects of light within it. They are arranged in two parts, with the first section showing the philosophical, theological and aesthetic background to the use of light, and the second section presenting descriptions of the observations made in specific churches. The methods which produce these observations are varied, ranging from a computer model which calculates the variations of light, to the descriptions of the researcher waiting patiently for the sun to rise so as to see the parts of the church which are the first places which the rays of the rising sun illuminate.

Most of the churches described are medieval churches, built between 600 and 1500 in the area of eastern Europe where Latin, Greek and Slavic traditions meet – or, as the editors express it, between the Balkans and the Baltic. Here theological and philosophical traditions have met to form a rich and diverse culture. There is the biblical affirmation of Christ as light and the ways that light reveals God. Then this idea is also developed in the philosophy of Plato and his successors such as Philo of Alexandria who distinguished between the natural light of the eye and divine light which can only be perceived by the intellect. This led a flowering in the Hesychast theology and spirituality, which flourished on Mount Athos in the fourteenth century, which taught the disciplines of the prayer of the heart with the aim of experiencing the uncreated divine light of the Mount of the Transfiguration. These traditions lie behind the creation of the church as a shared work of founder, architect, craftsman and painter to create a space which evokes a sense of reverence and devotion in the worshipper and so becomes an invitation to encounter the divine light. This brings together the two meanings of church as building and church as community of faith.

The church with its windows and openings uses the light of the sun to bring this presence of divine glory. There are different ways of achieving this, shown by the buildings described. Often, there are domes in the roofs of the Orthodox churches even if these intrude awkwardly into pitched roofs. There is an icon of Christ Pantocrator, reigning in glory, in the roof of the dome, and below that windows in the drum of the dome flood the space with light. So the eye is drawn from the shadow of the lower levels through ascending spirals of frescoes of saints and biblical scenes to the heavenly realm where Christ is enthroned. In some of the buildings the altar and the windows around it are carefully placed so that when the celebration of the liturgy is taking place the light of the morning sun shines onto the altar. Another approach is shown in a study of eight fifteenth-century churches in Estonia where the reserved sacrament is kept in oculi or niches in the wall. These niches are also windows open to the exterior, so that the sunlight shines from the outside onto the monstrance containing the sacrament so that it glows with light.

Other window openings are carefully placed to allow the sun's rays to give light and meaning to the church's worship. Several of the studies present the results of careful observation by researchers which show that there was collaboration between founders, architects and painters to create this space. The site would be carefully chosen not only as a place of beauty but as where the first light of dawn would reach through the windows. They were especially concerned that the sun should enter the church as it rose at significant turning points in the calendar, the summer and winter solstices, or the autumn and spring equinoxes. Sometimes this required precise adjustments during construction when the interior walls had to be re-oriented by a few centimetres so as to receive the first rays of sun and this resulted in the apse being twisted a little to the north of the east-west axis and becoming embedded in the north wall.

There are some carefully calculated effects of the light. In a Romanian church dedicated to John the Baptist, on his feast day, which coincides with the summer solstice, during the liturgy, a beam of sunlight from one of the windows moves across the fresco of the resurrection illuminating first the figure of Adam then moving to the hand of Christ grasping his, then on to the other hand of Christ holding a scroll with a text with the promise of the second coming.

Light is also shown through the painting of the frescoes. While modern western painting uses variation in colour to gain its effect, in these churches the effect is achieved through the varying degrees of light. In a church in northern Russia with paintings from the workshop of the iconographer Dionisy around 1500, there are bright colours used in the iconostasis but in the frescoes the painter relies on a small number of pigments, which are mixed with varying amounts of white limestone and then applied in thin layers with broad brushstrokes which gradually become lighter to give a shining diaphanous quality, which is then finished with a final transparent layer to give a glowing theophanic effect. This luminous quality in the painting adds to the character of the church as a place of light.

The final chapter describes research into Catholic mission churches built in California and Mexico between 1500 and 1800. Here too churches are built so that the rays of the sun fall directly on the altar, or other significant parts of the church, at the summer solstice. This glimpse into a different part of the world suggests that the light of the sun was widely used across the Christian world by church-builders to evoke this numinous experience.

The book invites future research to help us understand more about the use of sunlight in other societies and cultures. It also hopes for further studies into darkness, which is for Plato a form of potential light, and how lamps and incense go together with natural light to give a sense both of the movement from dark to light as well as the mystery of divine darkness. These all give further insights into the rituals and cultures of the societies which created and worshipped in these spaces.

Having begun with a personal memory, I will finish with another personal comment: I have seldom read a book so full of fresh insights, unexpected suggestions and discussions which have given me a new understanding and fresh appreciation both of the church buildings and of the meanings they express.