When I was playing the BG3 story using Tabletop, I did this very thing. Any time there was lore that needed to be made known, I had them roll either an Arcana check or a Religion check. That's what those skills are for, to see what the characters actually know about different things. If you fail the roll, you don't know, and if the player doesn't know, then the player doesn't get to know.
However, if you make the roll, the DM explains to the players what their characters know. So, when you first meet Shadowheart on the beach, whether she's wearing her armor or not, there should be a moment when you ask her who she is a cleric of. If she's wearing the armor, the DC for the Religion check should be REALLY low, like a 5. Shar is one of the most common goddesses of Forgotten Realms. Wearing her symbol should be a hugely easy thing for anyone in the world to spot. So, if they're going to have her wearing her armor right off the bat because she was on some top secret Shar mission and needed the extra custom armor for some reason, then most characters should immediately recognize her as a Sharite. Period. And the game should explain at least the very basics about Shar to the player.
And, for crying out loud, the armor is custom. It shouldn't just be some normal armor. She's a Sharite. Her armor should be customized so that it doesn't hinder stealth. Therefore, maybe she wore it on her mission to help her sneak and steal the artifact. That would give at least a reason as to why she's wearing it. If it's just regular armor some shmo in Baldur's Gate could pick up at the local flea market, then it makes no sense that she's flaunting Sharite symbols and then trying to hide that she's a Sharite, and especially that no one at all recognizes that she's a Sharite right off the bat.
And ESPECIALLY once you reach the grove and it tells you that you see a picture of Shar's symbol on the mural, and you recognize it at a Sharite symbol, characters should put two-and-two together and say, "Hey! Wait a sec! Shadowheart's wearing that same symbol all over her.
But if they put her in normal armor that doesn't scream Shar, they should have Shadowheart lie and come up with a different god or goddes of trickery so she can cast trickery spells and no one will suspect she's a Sharite right away. Then the first time you encounter anything about Shar, THEN the Religion roll should be made to see what you know.
I personally think this second path is the best. I mean, we already know she's a Sharite because we've played the game forever and we've researched it etc. However, new players would enjoy, I think, the mystery of uncovering her secret. Have her in plain armor and lie about her patron goddess. Then have the players find Sharite stuff and make Religion rolls to learn more about Shar. As the story progresses, they then begin to pick up on more and more hints from Shadowheart that she hates Selune and loves Shar. That would make the "Reveal" so much more meaningful, and the players would more likely go, "Hah! I knew it!" or "Holy crap! I did not see that coming. She's too nice to be a Sharite. Right?"
But again, the players need context for this either way. You can't just expect every player to read every book. Shoot! In BG1 and 2, I hardly ever read the books. They bored the crap out of me. Just give me the main story plot via dialogues and in-game events. Don't make me research and read crap all over the place or I miss major story elements. BG1 and 2 didn't do that. Most of the books and tomes were for lore purposes only. If the player wanted to learn more FR lore, they'd read all the books and tomes. If they didn't care, they'd play the game and still know the whole main story plot. If there was an important god or goddess in the story, the game explained them to you via someone explaining it.
