Okay, let me try to explain this without any spiritual terms whatsoever. It's going to lessen the impact of it, because those spiritual terms are important for thinking about it in the right way. They create the proper mental environment, strike the right "tone" for these things to be fully understood. Without them you're getting the "empty apartment" version to my fully decorated and painted pad if that makes sense.
Here goes:
As I said a buddha is not a supernatural being, nor is it a human being endowed with supernatural powers as the result of some esoteric practice.
In the past people believed this because it was an "expedient means". It was essentially a teaching given by someone who had reached this headspace in order to make it easier for other people to get there. People, especially ancient people believed in magic. This magical language was used, because the Buddha in question was a holy man coming from a Hindu spiritual tradition, and because the life state can FEEL like magic and can appear like that to outsiders.
Gradually a tradition evolved around this, in order to better replicate and expedite the process. Many thinkers within the tradition made contributions to it. Some were indeed fully-awakened Buddhas, some were people who got halfway there, and some were liars and charlatans.
However, over the centuries the practice was refined and monks like Nichiren worked to correct the improper teachings, practices, and undo the mistakes and lies.
The goal being to create and perpetuate a religious tradition that served people and helped us lead fuller, happier lives. To deal with our fears of death, mitigate our negative emotions and promote healthy thriving peaceful individuals and the societies they inhabit.
Overcoming life's difficulties takes self-esteem, humility, compassion, assertiveness and most of all a general refusal to give up and take the easier path that leads us further away from who we are and who we want to be. To maintain that attitude in the face of seemingly impossible circumstances takes faith in yourself - and not foolish arrogant faith that leads you to overestimate your own abilities or put down others, but grounded faith informed by both healthy self-esteem and an esteem and appreciation for others and for the world around you.
So how do you build this quickly and efficiently? Well humans like rituals and symbols. Exchanging money, getting married, daily routines, social etiquette, court - all rituals. Money itself is a symbol, so are deeds to houses, and legal contracts and a written language.
So, by utilizing these building blocks of human communication- a Gohonzon and a simple repeating chant (humans like patterns, repeat a simple phrase over and over, it will sound like music after the third or fourth repetition) a method for attaining happiness was created.
All these things together along with symbolic stories and community helps people attain their goals, feel better about themselves, treat each other better and become valued members of their communities while fearlessly pursuing their goals in ways that don't hurt others.
So Buddhas do exist and they are us. You don't have to be a Buddhist or know anything about Buddhism to become one, you just need to get yourself into a headspace where you realize you are one, have been one all along, and in fact all people are Buddhas, they just don't realize they have this potential.
When you look at people and yourself this way, it fundamentally changed the way you look at the world, which changes your behavior, moods and thoughts - all for the better and you achieve things you thought you couldn't begin to imagine, while having a blast doing so.
It's a mental "trick" that causes a positive feedback loop in the practitioner - except it's not a trick because what it really is, is a purposeful switch of perspective, and when you alter your perspective like that, it becomes true, because In fact it was true all along, you just weren't thinking of it that way.
No magic, no lies or self-deception, just head games intended to get you to interact with reality in a more positive mental paradigm.
Hence why people say Buddhism is more of a philosophy than a religion - BUT it's a philosophy that utilizes religious thinking and metaphor and ritual in a healthy way because it recognizes the power of these things to invoke emotional responses in humans and this their utility in building the headspace that Siddhartha Gutama lucked into while meditating under that Bodhi Tree: that is enlightenment.