First, nice graphs.
There has been sectarianism in most places, and notably against your brand of Christianity in England for several hundred years. They fought and killed kings to preserve it.
I count among family Catholics, Jews, Atheist and Protestants. We've managed to get along and I wonder why you cant.
The record keeping is pretty irrelevant from a faith perspective. Love God and his creation.
As someone who comes from a part of the world with the same social sectarianism as Northern Ireland (though thankfully not politically) I will comment on this. I was raised in one of the most Catholic towns in Scotland in a Catholic family. Every single one of my aunts married a Protestant in the 1970’s-1990’s, all Presbyterian with the exception of my dad who was raised Nonconfirmist Evangelical. This is quite a familiar pattern. I went to Catholic schools and while nearly all had a Catholic mother, half had a Protestant father of varying denominations. Why? The Catholic Church does not recognise (sacramentally) the wedding of Catholic who marries outside of the church. (One of the provisios for allowing Catholics to even marry non Catholics, which was never officially barred but not encouraged in Scotland until Vatican II) was to promise to raise any children as Catholics. This was of course duly done.
As a result of living where I live, my family, my schooling etc (and in part supporting Celtic FC) I am culturally Catholic. I’m an atheist of course, but culturally I am ‘catholic’ because Catholics in Scotland have always been progressive and liberal in comparison to the once Presbyterian majority. In Social Attitudes Survey’s here, Catholics are the most liberal religious grouping (excepting those who say they have ‘no religion’) when it comes to divorce, gay rights, equal rights and race relations. I am part of the ‘Roman Catholic community’
My dad for the record holds no belief and hasn’t since he was 15. Only my mum attends mass and none of her four children do. Me and my sister identify as atheist/agnostic. Out of 7 cousins on the Catholic side of my family, as far as I know only 2 attend mass one because he is only 8. This has never caused any problem in the family
With regards to the type of church that BRTD goes to, in Northern Ireland and Scotland such churches have came out of the Presbyterian or Non-Conformist tradition in Northern Ireland which is closely affiliated with Ian Paisley and in a nutshell are theologically close to the views that JCL espouses. They are therefore both the most socially conservative churches in Northern Ireland and also the least favourable to Catholicism and by extension power-sharing (more so than High Presbyterian or Episcopalians). So a Catholic joining such a church probably would have a hard time because it wouldn’t be so much about abandoning his faith, but rather opposing the cultural tradition of their community and joining a church that in many instances are instinctively anti-Catholic and anti-community