This paper provides a critical theological and historical examination of the Seventh-day Adventist (SDA) movement, with particular focus on the prophetic claims of Ellen G. White and the denomination’s doctrinal distinctives. Beginning with the historical roots of the SDA church in the Millerite movement and the subsequent Great Disappointment of 1844, the study explores how Ellen White’s visions shaped the trajectory of the movement and established her as its prophetic authority. The analysis identifies parallels between SDA teachings and ancient heresies such as legalism, Gnosticism, and Arianism, assessing them against the standards of biblical orthodoxy and early church tradition.