That would be the New Testament as without it Christianity wouldn't exist , merely Jewdeism.
The key figure of Christianity is Christ. For us to be true christian we must follow the work of Christ.
the Bible
The Holy Bible.
Jesus is Lord
A document controller is responsible for keeping documents organized and in correct place. The key achievement is making certain that these documents go to the correct people.
Key practices of Christianity are reading the Bible. Praying to God. Worship in whatever form you may. Attending a church to fellowship with other believers.
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Christianity is popular because it is The Truth and key to heaven.
St. Paul, who spread Christianity as far as Rome. Constantine made Christianity the official state religion of the Roman Empire.
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The constitution, and the decleration of independance.