I know we all want or hope that this party gets fucking destroyed or realizes how much they needed the priest but in reality, there is no way that is going to happen. We will have to see the hero party go up against various shit while the priest does all the hard work in the background which will really make this frustrating. However, I continue reading for the slight hope that they come back begging for his help.
This is not really a spoiler because it's so obvious but I'm spoiling it anyway.
The priest is the real hero, hes much higher leveled, physically stronger, and better informed. EVERYONE but the MC and the hero's party knows this because the man is super famous crusader. Besides the Hero's party the MC attracts ALL of the girls but he's completely oblivious to it. He just wants to do his job but the hero is a moron and no one with any authority tries to fix things EVER.
The church does have female priests that could go with the Hero's party but they don't want to because the hero looks like a lecher assembling a harem party. in actuality as most of you have guessed the hero is a girl and is hiding it because someone told her that people don't like female heroes but she actually wants to save the world anyway. But the hero is also a bad luck magnet attracting every worst possible demon boss in every place they try to level. The hero can't even fight properly because the hero armor she inherited is made for men and it painfully squishes her ample boobs. Every time they manage to get someone to fill in for their party they leave because they are all the most terrible people on the planet and the church keeps sending them in.
Meanwhile the priest is so OP he easily beats up the strongest of demons while handicapping himself supporting the hero party. If it wasn't for the fact that the Hero's unique composition is needed to kill the demon lord I don;t think the church would even bother doing anything but sending Ares in. At one point he disguises himself as a monster and infiltrates their underwater base by being so violent and over powered they have to invite him to join them. Meanwhile the hero continues to be underleveled, attract the worst attention, and fails at every quest because she is busy trying to hide her lady bits.
If the hero party turns you off leave now, they get worse and never become useful ever. It's just a sadistic story about a priest trying to secretly powerlevel some noobs but instead he suffers the worst setbacks and ignores every girl in the world who might have the hots for him. He wants to do a good job but the church won't tell anyone outright the best way to not fuck it up and lets everyone bumble around like morons in the hopes Ares becomes aware he's all they need and to maybe bring the hero along for the final blow.