Statistics on religion from the 2001 census in the North show a decline in the number of Protestants to around 53% of the population with Catholics now comprising nearly 44%.
It is the narrowest percentage point gap between the respective populations since Northern Ireland came into being in the early 1920s.
The Protestant population in Northern Ireland has dipped by 5%, but the rise in the number of Catholics, at 2%, is not as high as was predicted.
Belfast now has a majority of Catholics, as many Protestants move away to live in other parts of Co Down and Co Antrim.
Since 1961, when the Protestant figure was 63%, it has fallen in every census since then while the Catholic population has increased from 35%.
In the unadjusted data of the latest Northern Ireland census, those describing themselves as Protestant dropped to less than 46% compared with 40% Catholic.
It is understood 14% of people surveyed last year indicated no religion on their forms. Officials allocated 11% of these to Protestant and Catholiccommunities, based on certain indicators such as home addresses. Another 3% gave their religion as none.