I agree 100%, but I think the Ian rn wouldn't believe it unless he sees it for himself (this is just me assuming that's the case, based on how love triangle webtoons usually go)
I think it’s because Ian’s been mentally abused by the cardinal pretty much his whole life. Acting like his savior who only wants the best for him, but manipulating and molding him to his liking/into being dependent on his approval. He’s been punished to feel shame and guilt when he goes against him or for doubting his ”good” intentions. He’s been making excuses for the cardinal, both in his mind and to Sol when defending him. If Sol brings it up Ian’ll just feel the need to defend him and it won’t lead anywhere and just bring them further apart.
So, as the first person said in their answer, Ian needs to see for himself that the cardinal does not have good intentions in to believe it and I think Sol has realized it too. Ian do be doubting the cardinal, he left and choose Sol after all, but I think everything’s mostly the result of being manipulated since he was young and for such a long time. I kinda see it as who he was (Brian) is how he’d been if he’d not been found by the cardinal first if that makes sense? He feels inferior to Brian/scared that Sol’s love is due to someone he’s not anymore, but everything is all just the cardinals fault and Sol loves him simply because he’s him regardless, which he’s trying to prove/make Ian realize. It’s gotten through a bit but since the cardinal is still interfering it’s difficult.
coz Sol freakin want Ian to figure things out alone, he doesn't want to interfere with his decisions A FREAKING STUPID THING TO DO but I get it, he's a green flag, I ALMOST HATE IT!! WHY DON'T YOU FREAKIN CLAIM IAN FOR YOURSELF but doing that will be no diff than what that mf is doing so waaaaahh I want that mf dead
I don't understand why Sol can't tell Ian that the cardinal is a jerk...