The doctrine of the Trinity is not only central to Christianity but one of its most distinctive teachings. Although the term “Trinity” never appears in the Christian Bible, Christians believe the doctrine to be grounded in Scripture. The doctrine developed during the first few centuries of Christianity, as early Christians began to reflect on Jesus' teachings, the writings of the apostles, the sacred writings that Christians came to call the Old Testament, and Christian practices such as worshiping Jesus and the Holy Spirit. Given the difficulties surrounding the doctrine, debates and disagreements inevitably arose, leading to a need for church councils to set agreed-upon teachings. Within the boundaries set by the conciliar decisions, Christians have discussed important issues related to the Trinity, including what a person is, what natures and substances are, and whether the Trinity has implications for the claim that humans are created in the image of God. Since the early twentieth century, Christians of various traditions have paid renewed attention to the doctrine of the Trinity.