When considering the effect of the High Priest, you may say to yourself, wow, for two mana this can blow up any non-land my opponent controls! All that has to be done to it is have damage dealt to it. That doesn’t seem particularly hard to do, but on a 1/1, who’s going to take the time to block it or use a damage based spell on it?
In Limited, this card was particularly annoying if you kept it back to block someone’s non-flying bomb creature. In a sense, it has deathtouch, because as soon as it’s dealt damage it blows up something. So why not choose your opponent’s big creature? That being said, it seems that the High Priest was created mostly with Limited play in mind, which is no surprise as sets are designed primarily to function as Limited-friendly environments in and of themselves.
In Constructed, keeping back a couple of High Priests to block seems like a good strategy. I’m actually a bit surprised that this guy didn’t get more love at the time. Sure, you saw him played on the fringes of Standard, but there was no particular combination of cards that made him breakable. The only thing you could do, perhaps, was find a way to make him indestructible, so you could use his effect multiple times.
Actually, there was definitely a way to do this in the very same set: Boros Charm. It turns out, that you usually would use Boros Charm after a Blasphemous Act, which deals 13 damage to every creature. Sweet, right? But what you were trying to accomplish was deal 13 damage to your Boros Reckoners, which would them mirror that damage to your opponent’s face. Needless to say, that deck, along with having Vizkopa Guildmage as a way to give you lifegain and whittle down your opponent’s life was pretty good. It had those sorts of “oops I win” conditions that makes Magic so fun - or not so fun depending on what side of the board you’re on.
Alas, you’d much rather run Cartel Aristocrat in the two-slot to give your key creatures protection. High Priest’s effect is nifty, but while it’s a sweet attrition-based card, there was no room for it in the strategy. An attrition-based red/black/white deck never really took off with this card in it. So the High Priest became an under-appreciated, under-played rattlesnake card. In a format that wanted to sacrifice and not ping for damage, it just didn’t fit.
That being said, the High Priest basically was in the wrong Standard format. It definitely did its job in limited where white/black was a very solid strategy. Nowadays, it shows up in the occasional white/black Commander deck, typically alongside Teysa, Orzhov Scion and a bunch of other Clerics. It’s one of those cards that just never really found a real home outside of Return to Ravnica block Limited. Solid in design, the High Priest of Penance is now just one of those bulk rares that sits in a box somewhere wondering what could have been.