Of the 6,000 or so islands that make up the British Isles, few can be more beguiling than the Skelligs. Eight miles off the west coast of Ireland, they appear from the mainland as remote mountain peaks emerging mysteriously from the Atlantic. As well as drawing the eye, they draw the soul.

I yielded to their allure a few years ago and decided to visit the larger of the two, Skellig Michael. At the time I was researching for my book The Sacramental Sea, which examines the influence of the sea on Christianity and spirituality over the past two millennia.

What intrigued me was how this remote, inhospitable outcrop of rock could possibly have been home to a monastic community for 600 years. Why did monks settle there? What was it like to live, work and pray in such a place? Perhaps a visit would offer some insights.

I'd been warned that getting there could be tricky, and so it proved. For nearly a week my wife and I phoned a local boatman each morning, only to be told it was too rough to sail. Then, on the penultimate day of our visit, he said it was safe to go but we might not land. Fortunately, the sea's swell didn't stop us mooring on the island's jetty, and so we disembarked and made our way up the long, precipitous path that generations of monks had cut into the hillside. It led to the island's remarkable monastic settlement – so remarkable that it's a UNESCO World Heritage Site.

The ruins of the monastery on Skellig Michael are evidence of what happened when Christian monasticism spread from the Egyptian desert to Europe's Celtic seaboard, over a thousand years ago. Rather than searching for an inland desert, many drawn to the ascetic life in the West sought instead a 'desert in the ocean'.

Remote islands offered not only solitude but, more importantly, the conditions for spiritual testing. The earliest Celtic Christian monks drew their inspiration from Jesus' fasting and temptation in the wilderness - they weren't out to commune with nature.

Behind this attitude is an ancient cosmology associated with the Book of Genesis. This thought-world contrasted the relatively calm and tideless waters of the Mediterranean with the wild, tidal waters of the Atlantic. The Mediterranean was regarded as part of God's creation on the 'second day' in Genesis, when the primordial waters of the cosmos were separated to form seas. In contrast, the Atlantic was seen as the remnant of those primordial waters – the remains of the chaos out of which God created order.

This distinction is represented in ancient maps, which show the Earth as the three known land-masses of Asia, Africa and Europe separated by seas and rivers and surrounded by ocean. In this ancient thought-world, the British Isles and Iberian Peninsula were believed, quite literally, to be at the edge of the world – hence the former name of Fitzroy, the sea area to the west of Spain and Portugal: Finisterre - finis (end) terre (earth).

By locating themselves in these coastal regions, the monks believed that they were on the dangerous margins of God's creation - an ideal place for spiritual testing.

If what I've described is from a thought-world of a previous era, then I experienced a modern-day echo of it during another research trip. I was intrigued to discover that, not long ago, a monastery had been established on the remote, tiny island of Papa Stronsay in Orkney.

Again, I couldn't resist seeing it for myself, and so I island-hopped my way to the aptly-named Golgotha Monastery, home of the Sons of the Most Holy Redeemer who bought the island in 1999. It's hard to compare the sublime beauty of the stone 'bee-hive' cells on Skellig Michael with the white breeze-block buildings of Papa Stronsay. Aesthetics aside, it was fascinating to spend a day with a community who sought a 'desert in the ocean' in the twenty-first century.

The purpose of my visit was to find out what role the sea played in the monks' spiritual life. In one respect, I left disappointed. Their prayers and liturgy aren't full of maritime imagery, as I'd hoped; nor do they chant the Psalms to the sound of lapping waves, as I somewhat romantically imagined they might.

As became clear during our fascinating conversations, these monks aren't contemplatives drawn to be near the sea; they're following Christ into the wilderness. Like their ancient forebears, they're in search of isolation and spiritual testing, to be closer to God. For this community, the sea is a barrier - a moat to minimise wordly intrusion – and their watery surroundings heighten a sense of ultimate dependence upon God's providence. I travelled to Papa Stronsay hoping to find monks communing with their marine environment. Instead, I gained an insight not only into a strict form of monasticism today, but, possibly as well, into the minds of those who lived on Skellig Michael, centuries earlier.

Curiously, I'd half hoped we might get stranded on Skellig Michael - at least for a few days. I felt instinctively at home there, alongside the seabirds and archaeologists. In contrast, despite the warm welcome and kind hospitality at Golgotha Monastery, I was relieved when a monk ferried me back to Stronsay harbour. It was only when writing the final chapter of The Sacramental Sea that the reason for these contrasting feelings dawned on me.

The more I've reflected on the sea, coasts and islands, the more I've become aware of how my faith is grounded in a strong sense of God as Creator. In this respect, the magnificence of Skellig Michael and the wide openness of the Atlantic were rich spiritual nourishment.

In contrast, I felt constrained by monastic life on Papa Stronsay. This was intensified by feeling trapped by the sea, inducing a sort of spiritual claustrophobia. No doubt, if a time machine could take me back to the monks of Skellig Michael, I would have experienced something similar there as well – the feeling I had was more to do with the community's rule of life than their location.

I'm still processing what this means for me as a Christian and as a priest. Apart from the obvious conclusion that I'm not cut out for monastic life, it's exposed a tension within me that I suspect others share. That is, the difficulty of reconciling the innate spirituality that an encounter with the natural world can unlock in anyone - which feels universal and expansive - with living under the restrictions of the doctrines, practices and thought-world of one particular religious tradition. Both are routes to God: one broad and nebulous, the other narrowly focused.

My research trips involved, then, not only to two islands and two experiences of the sea, but an inner voyage, which continues today, exploring the somewhat dichotomous experience of being religious and being spiritual.

The Sacramental Sea: A Spiritual Voyage through Christian History by Edmund Newell is published by Darton, Longman and Todd and available to buy here