The Athanasian Creed is long and repetitive. Its insistence again and again that God is like this and therefore isn't like that can get a bit boring. It feels like a product of a fastidious age, when people spent far too long defining the finer points of theology and probably didn't spend enough time being 'practical' and perhaps neglected things like 'mission' and 'worship' because they were tied up credalising (not a real word).
In fact the point is simple, and highlights the major blind spot of our own age. God really exists, and because he really exists (rather than being a construct of a human mind), he exists in a particular way. If something is made up, you can think of it in any way you like; if something is real, then unless you conform your thoughts about it to the reality then you simply don't know it.
The God who is has revealed himself as Father, Son, and Holy Spirit - the three, and yet still the one God; in fact, the one God just as and only as he is the Three. Insisting that we think of God in this way is simply insisting that we treat God as a real. existing thing. In other words, it is a ridiculous demand if God is made up - and that God is made up is the first article of faith of the modern world.