Question

Which document standardized Christian beliefs?

Answer 1

"Among the writings considered central to the development of Christianity are the Pauline epistles, letters written or more accurately "dictated" by Paul of Tarsus to various churches. Many of these are now regarded as scripture. Some scholars think Paul articulated the first Christian theology: namely that all people inherit Adam's guilt and can only be saved from death by the atoning death of the Son of God, Jesus' crucifixion. General epistles, written by other hands than Paul's, circulated in the early church. Many of them, including one written as late as c 150, were eventually included in the New Testament canon."

So...I am thinking the New Testament?