The earliest indications of the doctrine of the Trinity appear in the _______.

The correct answer is the New Testament. The doctrine of the Trinity, which encapsulates the understanding of God as three distinct persons—Father, Son, and Holy Spirit—while being one in essence, is primarily developed in the context of the New Testament writings.

In particular, passages that reference the relationship between Jesus and the Father, and the coming of the Holy Spirit, lay the foundational elements for Trinitarian theology. For example, the Great Commission in Matthew 28:19 instructs believers to baptize "in the name of the Father and of the Son and of the Holy Spirit," which illustrates the unified yet distinct roles of each person within the Godhead.

Furthermore, verses such as John 1:1-14 establish the divinity of Christ, affirming His role as the Son within the Trinity. The early church writings and theological formulations that followed were influenced by these New Testament texts, as they sought to systematically articulate the understanding of God as a triune being based on the revelations found therein.

While the Old Testament contains hints and foundations for Trinitarian thought through certain plurality of God in creation and references to the Spirit of God, it does not explicitly teach the doctrine of the Trinity in the way the New Testament does.