The Holy Bible, the writings of the Church Fathers, & various other writers such as St. Thomas Aquinas.
The writings of the Church Fathers, The writings of St. Thomas Aquinas, The writings of CS Lewis, The writings of Alexander Solzhenitsyn.
The Bible very simply.
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Augustine sought to explore religious and philosophical questions concerning human nature, God, and morality. He also aimed to explain and defend Christian doctrine, particularly in response to the challenges and criticisms of his time. Augustine's writings provide a comprehensive framework for understanding key theological concepts within Christianity.
We have the ten Commandments, the Lord' Prayers. In short the entire bible is important.
The Roman historian Tacitus called Christianity a living torch. He also used the words Christ and Christians in his writings.
The Holy 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.
I don't think there are any. I would confirm.