To many Christians, she is considered a living saint, whose work with India's poor demonstrated a life dedicated to Christ's teachings. But, a new study from the University of Birmingham has explored the doubts and struggles Mother Teresa battled throughout her ministry and life, revealing a human side that many believers will be able to identify with.

The research, published in Critical Research on Religion, delves into the 'the dark night of the soul' that was revealed in the book Come Be My Light, an edited collection of her personal writings from 1929 to 1994 that was published 10 years after her death. At the time of its publication, the book caused surprise among both her supporters and detractors with its revelations that she experienced deep loneliness and struggled to find God in a world of pain and suffering.

"In Roman Catholic spiritual theology, the concept of 'the dark night of the soul' refers to a period of extreme spiritual agony that eventually leads to a complete mystical union with God," Dr Gëzim Alpion, Associate Professor of Sociology at the University of Birmingham and author of the study, said.

"Mother Teresa was always a controversial figure, but after Come Be My Light was published, this perception increased significantly, chiefly as it revealed a lifelong struggle with her faith, which disrupted the image many had of her - a devout and unwavering servant of God."

Dr Alpion. who has been researching Mother Teresa's complex personality for over twenty years, used both a biographical and a new sociological approach to examine the impact that this extended crisis of faith had on the work that Mother Teresa did in the slums of Kolkata among the poor and suffering.

The study argues that her desire to find God drove her to enlist numerous helpers including the poor, former pupils, nuns and volunteers. Teresa also struggled with feelings of guilt around her motivations for her ministry, admitting that her work with the poor was primarily 'for the sake of her soul', and revealing her discomfort with the idea that she was 'deceiving' those working for and trusting her.

"Teresa's struggles with her faith became her main motivator for her work, and this continued as she got older. We can see this clearly in 1953 when her 'darkness' had become so 'terrible' that she wrote that she felt 'as if everything' within her was 'dead'," Dr Alpion said.

"From then onwards, her decisions to expand her work across India in the 1950s and overseas from the 1960s were geared towards easing this pain."

These revelations about her ongoing struggles have raised questions about her mental health, with scholars split on the topic of diagnosing her with depression.

"Her defenders resist the idea that her struggles with her faith were symptomatic of depression, whereas a few admit that her writings display signifiers of a depressed person. This reluctance to acknowledge the link between the dark night and depression points to the bias and the stigma surrounding mental health within the Catholic and Christian communities," said Dr Alpion.

By the end of the 1950s, Teresa was seemingly resigned to living with this burden, writing in 1962, at the peak of her spiritual crisis, that were she to become a saint it would be one of 'darkness'. Hoping it would at least lessen in its intensity, she became more reticent in sharing about it from the 1970s onward, expanding her ministry into Communist countries (including her native Albania in 1989) and making this 'godless' part of the world her last hope for dispelling her doubts.

However, Dr Alpion does not think that the existence of these struggles should necessarily be seen as diminishing Mother Teresa's legacy of faith but, in fact, illuminates the true depth of her spiritual journey.

"To diminish Teresa's struggle with her faith is to diminish her efforts. This research outlines that her lifelong battle with her faith not only influenced her work but determined her choice of vocation and every decision after, including the charism of the Missionaries of Charity and the stages of her ministry," he said.

"If Mother Teresa achieved anything in her life, it was her ability to raise awareness of the sacred dignity of human life, unlike anyone else, spiritual or nonspiritual. This is no small feat for someone who seemed to be so tormented."