Curie was probably an agnostic, but certainly non-religious in a heavily religious society.
Curie was a revolutionary, a feminist and maybe a libertarian.
Marie Curie, born Maria Sklodowska, was born and raised in Warsaw, Poland. Due to her work with radioactive materials, Curie contracted leukemia and died of pernicious anemia in 1934.
Curie, being Polish and living most of her life in France, was surrounded by Catholicism and her mother was an active Catholic. Her father, who was a physics professor in Warsaw, is cited as having been a "freethinker," which is code for atheist.
Curie herself was not religious, and is often considered an atheist, but it seems more likely given her scientific worldview that she was more of an agnostic. She said:
Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.
Writing of her marriage to Pierre Curie (in a civil, non-religious ceremony), she revealed the secular nature of both Pierre and herself:
Pierre belonged to no religion and I did not practice any.
This may all seem rather vague, but when considering the highly religious environment of Europe during the 19th Century, for Curie to be so nonchalant about religion amounts to a bold statement against the social conventions of her time.
Science, women and politics
Curie was a bit of a revolutionary. During her youth in Warsaw, she participated in a student's revolutionary organization protesting Russian occupation of that part of Poland, and ultimately was forced to flee to Krakow (then a part of the Austrian Empire) to escape a potential backlash.
Curie was fiercely loyal to France in her later years and during World War I, she drove ambulances (that she had equipped with x-ray equipment for the French government) to the front lines of the war.
However, France was not always kind to her, and in her battles with the patriarchal scientific community there, Curie became a champion of early feminism. She was the first woman to get a Ph.D in research science in Europe, the first woman professor at the Sorbonne, the first woman to win a Nobel Prize and the first person ever to win two Nobel Prizes in different scientific disciplines. She certainly proved for many women that they could run with the scientific big dogs.
Despite her brilliance, Curie was denied a place in the French Academy of Sciences, with the stated reason being:
Women cannot be part of the Institute of France.
As a result, she wouldn't allow the Academy to publish her work for the next ten years.
Also, and I'm tempted to call Curie a libertarian (by modern standards of course), after her husband died, leaving her to care for their two children alone, Curie refused state welfare assistance. As to why she did that, details are a bit sparse. If any of you commenters would like to weigh in on this, I'd love to hear your thoughts.