When I knocked on the door of a convent in Dumbarton last week, I half-expected to be greeted by a human blur of black-and-white.

But I quickly discover that the nuns of Notre Dame wear twinsets and trousers. Their well-cut hair is worn loose and off their faces, not covered by a veil and wimple. In fact, the only visible sign of their vocation is the gold cross they wear around their necks.

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"Well, when did YOU last see a nun in a habit?" retorts Sister Kate Mulligan, when I remark on her appearance over tea at the convent of Notre Dame of Namur, which is about to be vacated by the order after 106 years. "I'm glad we don't have to wear it any more. The hems were always getting covered in mud, the wimple was a hazard for driving, and when you were out and about the only things you attracted were drunks and dogs."

Sr Kate, the local moderator of her congregation ("mother superior" and "orders" went out with the ark), was born in Strathaven and is an incredibly young-looking 71-year-old who, as she says herself, could easily be a granny by now. But she chose to leave behind any thoughts of marriage and children when, at age 23, she answered the call to devote her life to God and to helping others. That meant taking lifelong vows of poverty, chastity and obedience. "I used to think I'd get married, have babies, and that would be my life. But there was this force, this power that drew me into something that was very unknown, propelling me into another way of life, and there was a sense that I didn't have an option," she explains. "I felt carried by the grace of God."

She says that as a novice she enjoyed wearing the habit because it was a "sign of being part of a community", even though it also suppressed individuality. But now, NOT being identified as a nun actively helps her carry out her life's work.

The wearing of the long black-and-white habit was discarded by the Notre Dame congregation in the 1960s after Vatican II, which urged religious communities to develop a greater engagement with the modern world. Not all felt this was necessary: the contemplative Carmelites in Dumbarton and Kirkintilloch (who have purchased the Notre Dame convent and move in when they leave), and the Little Sisters of the Poor in Robroyston, who care for the elderly, have retained theirs. But the Notre Dame congregation, founded by St Julie Billiart in 1804 to work among the poor and the abandoned, saw the logic in adopting secular clothing and returning to the spirit of their founder.

In the early days the original convent, built in 1908 and now derelict, housed 30 nuns who all wore the habit when teaching girls at the neighbouring primary and secondary school. They moved to their present convent, another building within their vast grounds, in 1996. Last week, the last of the original sisters, many now in their late 90s, were relocated to a Notre Dame convent in Liverpool to be cared for by specialist nursing staff. But as we talk in the desolate atmosphere of the almost-empty convent, just days before the order leaves Dumbarton for good, the mood is determinedly upbeat. It's clear that Sr Kate and her colleagues possess an intellectual force as strong as the winds of change that are blowing.

The four nuns I meet each have double or triple university degrees and they have previously worked with the poor all over the world. Sr Kate's degree is in English and psychology and she works as a child psychotherapist at the Notre Dame Centre (formerly the Notre Dame Child Guidance Clinic) in Glasgow's West End. Sr Mary Kavanagh, born in Leeds, became a nun at 21 and is now 66. After a degree in theology she was a missionary in Peru for 23 years and now teaches literacy and does prison outreach and chaplaincy work. Sr Mary McClure, 62, was born in Plains, near Airdrie, and became a nun at 22. She studied education and theology, has a degree in psychology and has worked in South Africa, Nicaragua, Zimbabwe and California. She now works in Royston, one of Glasgow's poorest areas, helping women back into education. She has four nephews, two nieces and is great-aunt to six children. Sr Gail Taylor, the youngest at 51, was a chemistry teacher before gaining her PhD in psychology and psychotherapy, and she too works at the Notre Dame Centre, helping children with emotional distress.

Almost 100 years on from the Catholic Education (Scotland) Act 1918, they admit their original purpose – educating young women in Notre Dame schools and at the Notre Dame teacher training college in Glasgow, under the tenet that if you educate a woman, you educate a family – has been gradually subsumed by the state since the introduction of comprehensive education in the 1960s. The era of women having to resign when they got married has long gone, and Catholic lay women have become leaders in education as headmistresses and heads of department just as the nuns did before them.

But leaving the convent is not an admission of defeat; it's an acknowledgement that times have changed. There are fewer of them able to earn money for the upkeep of the convent and its grounds, and like many women today they all want to keep working even if some of them are past the traditional retirement age.

Life now for the progressive Notre Dame nuns means either living together in smaller groups, or alone in flats, close to where their work is. This is in order to have a presence in their local communities.

"Forty or 50 years ago the sisters were more homogenous, living in the same house, going to morning Mass together, teaching all day and having recreation time together in the evening," says Sr Kate. "Now our work is much more complex and we don't do things in a group any more.

"We don't have immediate family commitments, and we still have lots of time and energy to take on other ministries in other parishes. Our work in Scotland continues even if we no longer own the schools."

Given the dynamism and intelligence of these highly engaging religious women, I wonder how they feel about their place in the Church given the higher profile of parish priests – many of whom don't have degrees and have never left Scotland.

There's a silence. Then Sr Kate says: "We have a love/hate relationship with the institutional church in Rome. I'd like to see us take our place in the church. We enjoy a good relationship with many priests and there are kindred spirits among the clergy. But we've seen a lot of nuns leave because of problems with the male-dominated structure."

Sr Mary McClure adds: "You talk about women breaking the glass ceiling. Well, with us it's about trying to break the stained glass ceiling."

"I wouldn't want to be ordained in the present climate of the church, but at least it would give us a stronger voice," says Sr Kate.

Sr Mary McClure adds: "Theologically speaking, there's no reason not to ordain women. Sociologically there is, because of ongoing issues with women's place in society."

There is, as has been well documented, an ongoing decline in vocations to religious life. Why do they think so few young women feel themselves called to God? Once again, the sisters surprise with their readiness to grasp this uncomfortable nettle. "We left our families and others at a relatively young age, prepared to make a lifelong commitment to God and without thinking it through too much," says Sr Kate, who has two married nieces and one 18-month-old great-niece. "Young women today are taking longer to commit to many things, including motherhood and marriage; it's the nature of modern life."

Sr Mary Kavanagh agrees, pointing out that even if young women do opt to work with organisations like Voluntary Service Overseas, it's on short-term projects that don't require long-term commitment. Sr Mary McClure adds: "I wonder how they'd cope with the male domination of the Church. Young women are being educated to take their place in the world, to be anything and anyone. You no longer need to be a nun to get that."

Soon they will lock the door for the last time. There is sorrow, but it's not for themselves: "We're very sad for the older sisters who were asked to move," says Sr Kate. "They expected to end their days in Scotland. It was they who were asked to make the greater sacrifice.

"It's sad that we've lived in the area for over a century. But it's not tragic, because we're alive and well in other places in Scotland. Buildings are just buildings.

"Letting go means you can start life elsewhere."