What doctrine affirms God's existence in three persons?

The doctrine that affirms God's existence in three persons is the Trinity. This concept is foundational to many branches of Christianity and underscores the belief that God is one being who exists simultaneously and eternally in three distinct persons: God the Father, God the Son (Jesus Christ), and God the Holy Spirit. This understanding emphasizes both the unity and diversity within the Godhead, highlighting that while there is only one God, He reveals Himself in three co-equal and co-eternal persons who interact with the world and with each other in relational ways.

Monotheism, while it supports the belief in one God, does not specifically address the nature of God's existence in terms of distinct persons. Deism posits a creator who does not intervene in the world, lacking the relational dynamic expressed in Trinitarianism. Atheism outright denies the existence of any deity, which stands in direct opposition to the concept of the Trinity. Thus, the Trinity is the only doctrine among the provided options that accurately describes the Christian belief in God existing as three persons in one essence.