What is Christianity?
Christianity teaches that there is only one God and that "in the beginning" this God spoke the entire universe into existence.
On earth God creates two people, a man and woman, named Adam and Eve.
They are to tend the garden in which they live, and are free to eat from any tree except the Tree of Knowledge of Good and Evil.
Sometime later Eve is tempted by Satan and eats from this tree. Then Adam eats from it too. It's at this point that the two become aware, and "understand good and evil," evidenced by a new shame of their nakedness.
Disobeying God results in them being expelled from the Garden of Eden and suffering the effect of death.
Many generations after Adam, God chooses a man named Abraham to make into a great nation. A nation that will one day birth a Messiah to release mankind from the bondage of sin and death.
Our modern calendar splits time at the approximate date of the birth of Jesus who Chrisitians believe is the promised Messiah.
According to the NT, a collection of writings from early followers, Jesus is born to a virgin named Mary, lives a sinless life and then willingly sacrifices himself as the substitutionary atonement for the sins of mankind. The word atonement is used to describe an act that pays for or erases one's sins and transgressions. 3 days later, Jesus defeats death by resurrecting from the dead to open heaven to those who believe in him and trust him for the forgiveness of their sins. The resurrected Christ appears to many people over a span of forty days before he bodily ascends into heaven. From there he rules and reigns with God the Father and sends the Holy Spirit to teach his followers. It is also maintained that one day Jesus will return to this earth to judge all humans, living and dead, and grant eternal life to those who put faith in Him and eternal death for those who don't. Christians call the message of Jesus Christ the Gospel, meaning the good news.
And that is Christianity.