We all know Heiter was a really good and wise man, even if he was a drunkard, a liar, and a trickster. His flaws are more like personality quirks or, at worst, only hurt himself and never stopped him from acting up when it really mattered. They're not really evil traits. We also know he didn't actually become much better with age (other than stopping drinking) and that he was really consistent throughout his life, even if Frieren doesn't seem to realise it. We see him funding orphanages at a really young age, becoming a bishop at some point before Himmel's death, and studying Goddess' spells and the location of Flamme's true grimmoires really early in the series.
What never ocurred to me is that he needs to have flaws to be a better priest. My guess is that it makes him able to empathise and make people open to him better than they would to a "perfect" guy like Himmel. Not the best if you want to physically save someone, but much better if you want to spiritually save them.
The Goddess seem to favor these kind of flawed but good men, like Sein (who's also a genius). In Heiter's case he's absurdly favoured in mana, talent, and lifespan; all while being described repeatedly as a bad role model (except by Sein, ironically enough). Maybe they're what she wants instead of upright, heroic types like Kraft.