The Northern Ireland Statistical Research Agency released details of the religious breakdown of the province's citizens in Belfast on Thursday.
The Census 2001 figures show the number of Protestants has dropped to 53% of the population while the Catholic population has risen to almost 44%.
About 3% of people said they did not fit into either religious category.
CENSUS HISTORY
The last census 10 years ago categorised just over 50% of the population as Protestant and just over 38% as Catholic.
But given the number of people who chose not to state their religion, that has sometimes been boiled down to 58% Protestant and 42% Catholic.
The census figures have been the subject of intense scrutiny from Northern Ireland's politicians, with the strong link between religious affiliation and voting.
Many observers view the census as a gauge for a future poll on whether Northern Ireland should remain part of the United Kingdom.
The fall in the Protestant majority is less than what some nationalist politicians had predicted.
Sinn Fein chairman Mitchel McLaughlin had said the number would fall below 50% for the first time.
Speaking on Tuesday, he said: "It is understandable that unionists are nervous and unsure about the future given the demographic trend, but refusal to accept change will not prevent it."
Lagan Valley MP Jeffrey Donaldson said republicans would be "devastated" by the results.
He said: "A united Ireland is not even a remote possibility and it's time for republicans to accept that."
SDLP leader Mark Durkan said the figures painted a picture of a changing Northern Ireland.
"Nobody can say with certainty that this will lead to the united Ireland that the SDLP seeks," he said.
"But we can be certain that there will be a united Ireland if a majority votes for one and the south agrees. That is the position of the Agreement and it is the position of the SDLP."
Rising population
Besides the religious breakdown, a number of other statistics were revealed, including the number of people who say they can speak the Irish language.
The first details of last year's census were released in September.
They revealed the population on Census Day was almost 1.7m - a 5% increase over the last decade.
The figures also showed a higher proportion of young people under the age of 16 than the rest of the UK.
The figures also showed there were more women than men living in the province - with 863,818 females to 821,449 males.
While it had fewer people of pensionable age than the UK average, mid-year population estimates showed an increase of 7% in the number of pensioners in the province between 1991 and 2001.