If this secret pagan publicly professes the Catholic faith I don't see why he wouldn't be a member of the Church. If he was baptized as an adult, but internally rejected this act, even at the very moment of it's carrying out, he would be a member, a dead member.
I just clicked on this thread out of curiosity, and here is my first impression:
Talk about an ivory tower, hypothetical, useless, academic discussion! Is that what the rest of this thread is like?
In other news, "here is how many angels could fit on the head of a pin..."
Meanwhile, Tradition burns.
Seriously -- think of the situation for so many Trads today: the downfall of the SSPX, the necessity of Mass, the economic reality of 2018, newlywed couples trying to get by, couples where the wife is forced to work, the issue of education "how to school our children without sending them to public school, and I can't homeschool", finding friends for your children who aren't a horrible influence, young people discerning marriage (both male and female), dealing with non-Trad and non-Catholic family members, and so many other important, critical, AND PRACTICAL topics.
We're talking about issues that affect Trads everywhere, in all parts of Tradition, regardless of what group they patronize. There are a lot of struggles we have in common. Yes, even between a young sedevacantist couple and a young SSPX couple. Yes, I know that's blasphemy for those who love to divide...
So is this academic issue really the most important issue pressing on Trad Catholics today? Are our lives so stable and worry-free that we can indulge in such academic discussions? I'm sure many would like to say (or shout) MUST BE NICE.NOTE: I'm actually in a pretty stable situation myself -- but A) even I have issues I have to deal with, like the lack of weekly Mass, raising children in the 2018 pagan modern world, lack of friends/family "support network", etc. and B) I'm objective -- I don't always talk about myself. I step outside myself and my situation as easily as others are stuck thinking from their own situation/perspective :)
TL;DR:
We need less "Crisis in the Church" posts and more "Catholics Living in the Modern World" posts. The latter could have been called "The Rubber Meets the Road" or "Trad Catholic in Action" or "Trad Catholic life all week long - the struggles" and so forth.