The Holy Bible.
The key figure of Christianity is Christ. For us to be true christian we must follow the work of Christ.
The key to writing anything is communicating to the reader.
the Bible
Jesus is Lord
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.
That would be the new testament as without it Christianity wouldn't exist , merely Jewdeism.
Love
The first is being able to identify your audience. The second key to effective writing is establishing a purpose. The third key is formulating your message. The final key to effective writing is selecting your style and tone.
The New Testament is the Christian scriptures.
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.
A key's face is the top portion of the key where the letter, number, writing, or symbol is.