Yeah I mean most people call the thing "founded" on the Old Testament Judaism. There is something founded on the teaching of Christ, which has a background of Judaism, but is different, that was promoted by the Apostles (including Paul). Then there is the stuff that happened with the Roman Empire, the middle ages, the Reformation, and more recently the Great Awakening.
The point most people cite as the "founding" of Christianity is the Pentecost immediately after Jesus ascension. The accounts of this event in Acts are pretty unique: it was a spreading of the story of Christ and his resurrection, simultaneously, to a multinational, multi-ethnic, and multi-gender group. This is essentially unprecedented in all of religion: the 120 or so people who received the tongues of fire included women, "sinners", educated, laypeople, etc. No "select group". The initial message that reached thousands (if 3000 were "added to their number" then way more than that heard it) was done in multiple languages, simultaneously - no people group was therefore able to say "hey look we got it first!"
In a historical context this is radical - even contrasted with the Old Testament, where usually there was only a single "anointed" person at a time (Moses, the Kings, the Prophets). Instead you get 120 people "anointed" by the Holy Spirit all at once. They then spread it out essentially across the entire known world (in contrast to the specificity of the Israelite people group in the Old Testament). It was markedly an inclusive message, not an exclusive one.
So regardless of what history did with it after that event, that initial event was about the most inclusive thing ever, something arguably never seen before or even since. That event and the first sermon in Acts 2 - it's really a "for everyone" thing. That's why it was called the "good news" - so even if you think it was made up, that's the most astonishingly progressive thing to make up you could imagine.
That's one of the reasons why I feel so sad when I hear people today complain (and often rightly so) that "Christianity" has become bigoted and exclusive and hateful. Because it definitely didn't start out that way - with everyone sharing everything, in unity.
Side note: I'm still... struck by the fact that there is this idea that the Old Testament God is different from the New Testament God. If you take the OT and NT as a whole, not as a set of different stories, there is always this common thread underneath all the "fire and brimstone" of "Hey humanity (and often, specifically Israel), you keep trying to earn your salvation. If you want to try and earn it, fire and brimstone is what that looks like. That doesn't work people - so I have a plan and I will redeem you." Then Jesus comes, and does redeem all of humanity. Not under the law (which is fire and brimstone) but under grace. That's the story that stirred the hearts of those initial 3000 people. It wasn't a story about "God is going to conquer Rome" or "God is going to give you a comfortable life." It was merely the story that God has done all the work to get people in a restored relationship with God and ultimately conquered death itself, so we don't have to worry about "did we do enough good?" while we're alive.
Not having to worry about that is, some of us think, a great freedom. Not to justify any action we may take, but to not be fearful that our mistakes will irredeemably destroy our relationships with other people or, ultimately, God.