Her best-known work is Kristin Lavransdatter, a trilogy about life in Norway in the Middle Ages, portrayed through the experiences of a woman from birth until death.

When she was only 11 years old, her father, the Norwegian archaeologist Ingvald Martin Undset (1853–1893), died at the age of 40 after a long illness.

She had put aside the Middle Ages and had instead produced a realistic description of a woman with a middle-class background in contemporary Kristiania.

[5] The title was Fru Marta Oulie, and the opening sentence (the words of the book's main character) scandalised readers: "I have been unfaithful to my husband".

Thus, at the age of 25, Undset made her literary debut with a short realistic novel on adultery, set against a contemporary background.

The first is about a woman painter who, as a result of romantic crises, believes that she is wasting her life, and, in the end, commits suicide.

The other tells of a woman who succeeds in saving both herself and her love from a serious matrimonial crisis, finally creating a secure family.

After short stops in Denmark and Germany, she continued to Italy, arriving in Rome in December 1909, where she remained for nine months.

The encounter with Southern Europe meant a great deal to her; she made friends within the circle of Scandinavian artists and writers in Rome.

She had considerable polemical gifts, and was critical of emancipation as it was developing, and of the moral and ethical decline she felt was threatening in the wake of the First World War.

[8] After the birth of her third child, and with a secure roof over her head, Undset started a major project: Kristin Lavransdatter.

She was at home in the subject matter, having written a short novel at an earlier stage about a period in Norwegian history closer to the Pre-Christian era.

Undset experimented with modernist tropes such as stream of consciousness in her novel, although the original English translation by Charles Archer excised many of these passages.

Subsequent translation of the Hestviken tetralogy by Nunnally are retitled Olav Audunssøn (1):Vows (The Axe), …(2) Providence, (The Snake Pit), …(3) Crossroads (In The Wilderness), and …(4) Winter (The Son Avenger).

Both Undset's parents were atheists and, although, in accord with the norm of the day, she and her two younger sisters were baptised and with their mother regularly attended the local Lutheran church, the milieu in which they were raised was a thoroughly secular one.

The crisis led her from clear agnostic skepticism, by way of painful uneasiness about the ethical decline of the age, towards Christianity.

It is noteworthy that The Master of Hestviken, written immediately after Undset's conversion, takes place in a historical period when Norway was Catholic, that it has very religious themes of the main character's relations with God and his deep feeling of sin, and that the Medieval Catholic Church is presented in a favorable light, with virtually all clergy and monks in the series being positive characters.

[citation needed] The attacks against her faith and character were quite vicious at times, with the result that Undset's literary gifts were aroused in response.

Undset's essays about Elizabethan era English Catholic martyrs Margaret Clitherow and Robert Southwell were collected and published in Stages on the Road.

Undset then threw a copy of Chesterton's The Everlasting Man on the manager's desk and exclaimed, "This is the best book ever written!

With a minimum of camouflage, it tells the story of her own childhood in Kristiania, of her home, rich in intellectual values and love, and of her sick father.

Her eldest son, Norwegian Army Second Lieutenant Anders Svarstad, was killed in action at the age of 27, on 27 April 1940,[14] while defending Segalstad Bridge in Gausdal from German troops.

[16] In 1940, Undset and her younger son left neutral Sweden then crossed the Soviet Union via the Trans-Siberian Railroad before arriving as a political refugee in the United States.

She was buried in the village of Mesnali, 15 kilometers east of Lillehammer, where also her daughter and the son who died in battle are remembered.

The Roman Catholic Diocese of Oslo in close collaboration with the Dominican Order in Norway have started initial steps in investigating and opening a possible sainthood cause for Sigrid Undset.

[23][24] Sigrid Undset and the plot of Kristin Lavransdatter are important elements in the 2006 Academy Award-winning animated short film, The Danish Poet.