Well, I'm not Christian at this point in my life but I've
noticed through the years that many current atheists were raised
in homes that were as intense about the version of Christianity
as they are now in their anti-theism.
In short, the direction changed, but the intensity remained.
I never had that intensity.
In theory, some basic Trinity knowledge should at least be
present through upbringing. That's an assumption on my part of
course and even if not, _some_ understanding of it is easy
enough to obtain simply LIVING in a Western society, just as
it's easy to obtain a basic level of understanding about Zen
Buddhism in the form expressed in the West.
Cultural knowledge.
But you are correct; it _is_ a hard thing to understand, just as
QM is a hard thing to understand. In both cases, somebody else
did the heavy-thinking and we just have to accept it and accept
trusted authorities, unless we manage to understand it, in which
case we don't need the authorities for support as much anymore
because we 'got it'.
Someone else made the math formulas. We just accept them. etc.
Now, I don't read the math, but I understand the process of QM.
Explaining it? Well, it'd take a lot of drawing and I can't draw
well and words are equally hard to describe it with. It's easy
when it 'clicks', but explaining it quickly? Not so easy.