How would someone describe Christianity?
One of the three great monotheistic religions (along with Judaism and Islam), Christianity began with the birth of Jesus. Believed by his followers over the centuries to be the son of God, Jesus was a Jew who lead a movement in the 1st Century dedicated to spreading and inculcating into the minds and hearts of people the basic foundations of a moral or virtuous life. As the tenets of what became "Christianity" spread and took root, it was eventually adopted as the "state religion" of the Roman Empire by Emporer Constantine in the 4th Century.
The defining characteristic of Christianity is the belief in Jesus as the son of God and as the savior of mankind, and that Jesus's return will herald the final judgement to be rendered upon humanity and the establishment of the Kingdom of God. Christians belief, as in the Holy Gospel, that Jesus was crucified by the Roman governor Pontius Pilate, was resurrected from the dead and "seated at the right hand of the Father."
While the above represents the shared gospel of Christianity, the history of this religion, as with others, subsequently became fraught with divisions, most notably, with the Reformation.