Well... There are so many plotlines going on JTA that ONE message would feel a bit like a downplay... But saying all of them would probably lead to a 10 page long post about each and every character.
The core message of JTA is about growing and acceptance of that growth...
Accepting that sometimes even when we are old and supposedly wise, we still make the same mistakes, as we are flawed by nature
Accepting even if we try so very hard, there are some things that are too hard for us to tackle on our own,
Accepting that we are not all alike, that our experiences as people are collored by our own views and prejudices, and that our mind can play tricks on us and even flat out destroy us from the inside if we do not learn and cultivate empathy and gratitude.
Accepting that even the largest and strongest among us are small and weak, learn that we need to work together, as well as hold our own weight... The differences are what make us work so well alongside one another;
And finally... Accepting forgiveness... Both for our mistakes and the mistakes done onto us onto us, the importance of letting go of the things that hurt you.
Growth is a never ending thing, we grow as people as long as we live, and so does JTA, the characters, the world, and the reader with it.
The core idea is that it looks like one of those 'power fantasy' series where everyone will rise up and fight god at some point, but when you look at the cracks beneath the surface of that 'grand adventure' motif, is when you see what it is all about.
Also... Another message is that maybe... You should take some time out from the cruel world you live in, and enjoy the company of a few friends ^^
https://tapas.io/series/Journey-to-Ataraxia