Because no one is saved outside the church, and because God gave His apostles the order to preach the gospel to all nations and baptise them...
..Paul, himself a convert, goes to Greece.
While in Greece he arrives in a city in which silversmiths have made large statues of Diana. They claim the statues are a God and worship them.
Paul, forgetting himself after taking an illicit drug known as "V2"
promptly greets the silversmiths with a warm handshake.
Then he gives them a relic of the saints, and allows the silversmiths to bless him. A committee is established by the apostles who also took "v2"
The committee intends to change the liturgy to make it more acceptable to those outside the church. Diana is no longer called an idol.
Priests ordained by the apostles who call this heresy are excommunicated from the church.
Meanwhile the other apostles have been in other places meeting Buddhists and hindus. A world council is held in Jerusalem which is the base of the church with all religions side by side.
Later the apostles grow old and die.
Having converted no one, and lost converts to Dianism,
the church disappears and is forgotton by all.
Christ returns and finds no faith on earth, and sends the whole planet to hell.
End of story.