How is first order logic complete but not decidable?

Question

I know that there are two different notions of completeness and that we shouldn't be surprised that a theory might be complete (in the sense of the theorem of completeness) but undecidable (so incomplete in the sense of the Incompleteness theorem), but I still have a hard time wrapping my head around those two different notions. Could anyone explain clearly how in particular first order logic is complete but undecidable ? And how in general this makes perfect sense for a theory to be complete but undecidable ? Thanks a lot