There's a quote out there that says "Tifa has complex feelings towards Aerith as a woman", and people say that she's jealous because Cloud hides in Aerith's church instead of talking to her.
Which makes sense to me. She’s not exactly jealous because she thinks Cloud is pining for a dead woman, she’s jealous that he’s more willing to take his troubles and stew in them in a place related to Aerith vs talking about it with her.
If I were to go to a place where me and close girl friend used to hang out vs talking about my anxieties and troubles with my partner, I couldn’t blame them one bit for feeling some type of way about it.
This reminds me of how one of the most popular CA fanart from the 2000s is a flip on the scene in CoT where TIFA is crying at Aerith's grave and Cloud is comforting her. But in the fanart, CLOUD is crying and Tifa is comforting him/looking conflicted. And it just goes to show how much this fandom loves to twist things around and be weird about it lol.
Tifa and Nanaki were the people who were closest to Aerith. Tifa believed that she and Cloud understood each other in regards to Aerith and would be there for one another in their mutual grief--it's why they all wear the ribbon on their arm, after all. But when she finds out that Elmyra asked Cloud to take flowers to Aerith's grave and he went without telling her or asking if she wanted to go, she's deeply hurt by that and that's what begins the downward spiral and doubt.
It's not even jealousy, really. If your partner went to your mutual friends grave without you and decided they don't want to talk to you about it, wouldn't you start wondering what happened to make them shut you out? Especially when they were openly shouldering the burden of that grief with you before. It makes Tifa wonder if she did something wrong. Then, Cloud leaves entirely, isn't picking up the phone, etc. but she finds out he's been sleeping at the church and that he's sick.
The scene with Tifa and Marlene in the church isn't complicated. Tifa holds no jealousy in her heart toward Aerith. She doesn't believe that Cloud went to the church because he's in love with Aerith or pining for her. She tells Marlene he went there because he "wants to fight alone" before amending her thoughts and realizing he went there because he
doesn't intend to fight at all. He went there because he's given up entirely and he didn't think anyone would find him.
The church is fascinating because people often refer to it as "Aerith's church" but ToTP makes it clear that the church itself has a power that goes far beyond Aerith. It was where Ifalna was going to take her because even she could feel it's power. The flowers grow there, not because of Aerith, but because of the planet. It's a symbolic place for healing for multiple characters--Ifalna, Aerith, Zack, Cloud, Marlene, Tifa, and even Kyrie.
