The base tolerance to heretics of any religion is -2, but as catholic you get another -1 on top of that.
That means you need +6 tolerance to heretics if you want to stay as a catholic and not bother about reformation.
You can get:
+2 from NI
+1 from 100 Legitimacy
+1 from Declaration of indulgence
+2 from Edict of Nantes (from the Wiki, so I hope the information is reliable)
TOTAL: +6
So, assuming you don't take any decision lowering tolerance to heretics, you should technically be able to stay catholic all the game without taking Humanist. And, as far as I know, only France is able to pull that off.
Bohemia, Burgundy, NED, Knights, Lithuania, Palatinate, Pomerania, Poland, Saxony, Ruthenian nations all have +2 heretic tolerance or more.
With humanist, anybody ignore the reformation by simply switching off Catholic too, and that's probably the easiest way to play Europe in terms of religion, though swapping to whatever center is near you works too.
France gets screwed over by the rare railroad operating against it though, if it gets that event. IMO there's no way a nation with perfect unity should have a religious turmoil event of any kind, but w/e.
