I'll just say that I read about it on slashdot in July 2010 and thought it was a cool project from a tech perspective with a reasonable chance of changing the world by providing protection from statist authoritarians.  It really wasn't a question of fiat pricing, but that it was an interesting project from a computer science and engineering perspective initially.

Reading some of the early writings on it (e.g. there was one that was something like "bitcoin is the economic singularity" online around a year after I first read about bitcoin) and thinking about it increased my thought that it could really make a difference and eventually help a lot of people, increasing demand. 

For me it was a combination of things.  I've had my bank accounts (wrongfully) frozen by the government which took court cases, time, and money to fix.  I've had chargebacks on paypal by scammers that cost me losses in business deals before.  I've been de-banked for no reason.  I've had to make international payments that required weeks to clear or even money orders sent through the mail along with worse things that Bitcoin can help protect against. 

I guess you could say experience.  When I found Bitcoin it was common sense that it would be the next form of currency.  It's a shame it's been crippled by bad actors over the years to be less useful, but I think it's still pretty cool for those who do use it.

I am not a bitcoin OG but that doesn't make me unqualified to answer the question from experience.
My belief in Bitcoin stems from it being a way to escape poverty and achieve financial freedom faster than the millennials and boomers. No offence to them.
Man, I don't want to be poor or work for 35 years before I can have the money to be free to do what I want, how, and when I want it. Bitcoin shortened this for me.

And that is the faith that I have in bitcoin that made me buy my first bitcoin after seeing how it has transformed the lives of the young people around me in all ramifications. Mind you, I didn't but it when it was less than $10k but when I did buy, it was the best decision I could have made earlier.

When I was new, I didn't think that the day of $100k would come. To be honest, I was just trying to get some for myself and with my ignorance, I am also converting it quickly when it's dropping.

That is for sure that the day will come for Bitcoin that it's price will reach that number you've mentioned. It's only a matter of time and we're still early if that's the price target of yours. And some others that don't have it yet, they have to start thinking when they will buy or they should DCA as early as now.

Let's be honest most of them never really believed in Bitcoin. Some bought it because they want to try their luck in the digital financial system, many owned their Bitcoin through mining, earnings, airdrops, bounties, faucet, etc. As they were able to have handful of it since it was very cheap then they left as it seems worthless not until the price went up their believed changed.

At first, they wanted it to be for transaction purposes only, but with time the mindset changed after Bitcoin kept breaking a new ATH. In general, a few Bitcoin OGs by luck and opportunity not because they had faith in Bitcoin then.

I even can not know what I "believed" back then. Belief is a mess. It is not a pure moment when you go: yep, I believe now. You see the cracks, you sense economic uncertainty, you check the inflation figures and something in your gut tells you this cannot last. If you're wondering if you should sell your car, that whisper will soon become a hum, and then a scream, and you'll realise you're in Bitcointalk with 47 tabs open.

Saw it go to $20K. I felt I was a genius. Then saw it tank. Thought I was an idiot. Then saw it return. Thought... well, maybe I'm not completely insane. That cycle repeats. A lot. Was I "knowing" it would come to $100K? No. But I thought there was nowhere else rational to park confidence. Fiat? dishonest. Real estate? Bloated. Stocks? Overstimulated. At least Bitcoin has rules.

Seeing the world becoming louder and dumber and Bitcoin simply not caring convinced me. It just kept ticking blocks. No marketing budget. No boss. That’s... grounding. What if most HODLers lack even basic knowledge of what they are carrying? Mass adoption could make things worse rather than better since the philosophy dilutes as the cost rises.

Perhaps that's good. Perhaps the goal these days is not purity. And Bitcoin still feels like the only non leaky lifeboat available. Perhaps right now I believe more than I did years ago. Perhaps I'm still not entirely sure what that belief actually means.

I don't have faith when I first knew about it. I just keep trying to accumulate Bitcoin because I can get it by free from faucet. I'd say to myself, "Well, while it is free so I can claimed every day without paying any money and sell it to have money."

That is what I did and yes, I earn first money from Bitcoin at that stage and still accumulating more as the reward still good for me. I don't think that the price will become $100k some day but that is happen before we see Bitcoin price down now.

What I believe from Bitcoin is I can make money from my investment and other ways. We will see the bullish market come although we don't know for sure when that will happen.

I believe most of us started out coz of the FOMO .. and the hype at that time was something we call viral today.. I am talking about the very early times.
Wasn't of legal age in that era... but most of my connections that were going for Bitcoin were those who were already into stocks and trading... so I think it matched the trend and most of them thought that it was the same genre.

But my faith today.. AS YOU ask for BTC is - the early bird caught the worm!!!

Those old and inactive addresses can belong to either people who lost their bitcoins forever, who already died, or people who are still holding their bitcoin and have access to their bitcoins.

Sparsely we see whale alert notifications about very old Bitcoin addresses wake up and make transactions. We never are sure about old inactive Bitcoin addresses won't wake up in future.

The Bitcoin HODL waves can show you old inactive UTXOs. About 8% of Bitcoin UTXOs are inactive for 7 - 10 years.
https://www.bitcoinmagazinepro.com/charts/hodl-waves/

What happens to lost bitcoin?
Lost Bitcoin: 3.7 million Bitcoin are probably gone forever.

When Bitcoin was created I didn't think that it will last, I could have been very rich by now if I did, but like most people I didn't take it seriously, I never bothered to make a thorough research about it then. It is better late than never and I'm in now, I believe that with my DCA consistency, in the next two and three circles I can become among those that will have many returns on their Bitcoin investment. With the rising of Bitcoin adoption I know that it will make many people rich on the long term, maybe then OP can still repeat his question about what our faith was when we were buying in 2025 at $83k.