My closeted friend

Hey, this is my first ever post on Reddit, so sorry if I make some stupid mistake(s). This post is really long, I realise that, but I would really appreciate it if you took a few minutes out of your day to read it. Thanks.I think some background information about me might provide some context for this post. So here goes, I'm a straight male. I was raised religious, but I'm not anymore, and the religious oppression of gays is what made me start asking questions about my faith and retreating from it. I haven't told my family that I'm non religious, so in some ways I can relate to being a closeted gay, though I realise the obvious differences between my situation and one of a closeted gay person. But you can see how this is a topic near and dear to my heart.So, finally getting to the main point of this post, I have a friend who I strongly believe to be bisexual. We'll call her Sarah, but that isn't her real name.A long time ago, I overheard Sarah talking to a friend of hers, and of mine. We'll call her Julia. They were saying something about a secret, and of course I was curious about what it was. They refused to tell me, but I heard something about them messaging each other about it.Alright, this is the part of this story which I regret. I swear I regret it, and I wish I hadn't done it. Me and Sarah are good friends, so we know each other's phone passwords. Me, Sarah, Julia, and a few others were out at an amusement park, and everyone went on a ride except for me. They all left their bags with me, so nothing would be stolen. I looked on Sarah's phone at her messaging history with Julia, and of course I found the secret. The conversation went a little something like this:Phone call Julia: So, you're bi? Julia: Who knows?Sarah ignored those questions, as I assume they talked about it on the phone call maybe. A different conversation started after that.I know I shouldn't have looked on her phone, but I couldn't just forget what I knew. So I made a plan. Later that day I took Sarah's phone, playfully, and in front of her opened it and went into her conversation with Julia. She took the phone away from me at that point after exclaiming to Julia "He's going into our conversation!" with a panic-stricken expression, and a knowing look passed between them. My plan worked, she saw me look at her conversation without seeming like I had looked at her phone while she was on the ride. It made me seem like I stumbled onto it without trying to, and like I wasn't a creep. Sarah still didn't think I saw the secret, but she now knew it was a possibility.After we all went home I texted Julia that I knew the secret and that I had to tell Sarah, it was only right. At first Julia denied it, but eventually she admitted that I was correct. She didn't want me to tell Sarah I knew, but eventually agreed that I probably had to. It was late and I didn't want Sarah to stay up worried about it, so I waited to text her about it until tomorrow.Ironically, I texted her about it on the way to church. And her response almost broke my heart. She said, and I quote, I still have the text, "No there is nothing I was just talking about random stuff". And then, the next text was a semi incoherent dismissive statement that it was a girl thing and that they were just talking about random things. I responded with "Oh ok" because I could tell I was making her uncomfortable. But it doesn't add up. Everything else clearly points to her being bi, Julia admitting that I was correct, the quite clear text from Julia to Sarah that blatantly states that she is bi.So I just don't know what to do. I know coming out, even to just a few people is very hard. I get that. But I've always been very supportive of gays, and she knows that. I just wish she felt comfortable enough to come out to me, especially when pushed like that.I just don't know what to do anymore, it's hard to support someone when they won't let me.TL;DR: I am very confident that my close friend is bisexual, but when I told her that it's ok and I support her and I won't tell anybody, she denies it. What should I do?