The Serious Fallacy At The Heart Of Modern Christianity

I'm putting this down primarily for my own use and perhaps so other people can get something out of it. I have tentatively named the fallacy at the heart of modern Christianity 'Conditional Prospective Substitutive Atonement'. So what does all of that mean?

Abbott's Hogwarts campus (St Patrick's Seminary Manly)

The 'conditional' aspect means the rest of the proposition is conditional on whether a person accepts the proposition or not. More on that later.
The 'prospective' aspect means that the idea or offer proposed by Christ is not 'retrospective' but 'prospective'. In other words, it only applies to those born at the same time or after Christ. What Christ proposed, as interpreted by many, if not all modern strands of Christianity, is that Christ substituted his atonement for the sins of potentially billions of people who were born after he died. This is the 'substitutive atonement' aspect of the fallacy. It relies on 'original sin' or the idea that babies are not innocent in they eyes of the church, and that their inescapable sin can only be atoned for by a deity that died thousands of years before they were born. 'Substitutive atonement' can only occur if believers accept the premise of 'substitutive atonement'. That means the concept can only be validated by itself. Presumably, another form of atonement must have to take place if the 'substitutional' option is not taken-up. There appears to be no test of whether this atonement has occurred or not. So there it is. Prospective Substitutive Atonement is Conditional on whether a person thinks it will work or not. If you don't accept it then you will have to atone for your own original sin, or so they have been telling you for two millenniums.
The fallacy raises some interesting questions. Why did millions or even billions of people born before Christ have to atone for their own sins, while those born after were offered the substitutional option?
Why are human beings born with sin but not the higher primates or all the other life forms? Does this make animals purer than people?
Why did a creative force in the universe make being born with sin so abundant for humans, yet make being absolved from sin so narrow and monopolised?

Isn't this a tell-tale psychological remnant that the proposition was devised by men and not by God?
'On special this week only, hurry in while stocks last' is another psychological sales pitch similar to 'substitutive atonement'. The major fallacy at the heart of Christianity has existed for a surprisingly long period of time. Now a critical mass of literacy, logic and digital communication has exposed it for the lie it always was.