So I’m addressing this in two parts.
First, the “effect” so to speak, is
as though you had cast the same spell yourself
This clause covers the spell, so knowledge is not required for this to work. So if you can use Spiritual Counter, it would work.
Second, the feat has a requirement for its use, including
be able to cast that spell as if it were on your spell list
This wording is very strange, but what it seems to be saying that the requirement is that, if this spell were on your spell list, you’d be able to cast it (because you can cast spells of that level, and it doesn’t conflict with your alignment or whatever). It doesn’t say you have to know it, or even have it on your spell list; it just says you have to be able to cast it if it were on your spell list and you knew it.
But the wording is awkward and I can’t claim that this is absolutely, 100% what it meant. It’s just the only interpretation that I see that fits the words we have. If you were required to know the spell, it would just say that instead of the weird thing it does say. Presumably, they phrased it this way to make the feat reflect any casting prohibitions you have, like cleric alignments or wizard banned schools.
Honestly, I’d probably just ignore that requirement entirely, and make the feat just work when you spend enough turn undead uses. Even if you could never cast that spell, or can’t cast spells at all (I’m sure there’s some way to get turn undead without spellcasting). You have to burn a ton of uses to activate the feat, and counterspelling is a really inefficient tactic. I’ve played dedicated counterspellers (because no one who isn’t dedicated to it is going to do it), and in my experience, it doesn’t work very well and isn’t very fun. Worse, strictly speaking, options for immediate-action counterspelling (the only counterspelling worth considering) don’t apply to Spiritual Counter. Readying an action like that is very painful. And preventing a cleric from countering a spell he could not cast (because it conflicts with his alignment) seems absolutely silly: countering spells of opposing alignments seems very much an act in line with his own alignment!
Anyway, also, clerics do not “know” any spells; all of their spells are divinely granted to them each day. They can prepare any and all cleric spells up to their maximum level, without any need for prior knowledge.