Marcellus of Ancyra was a 4th-century Christian bishop and theologian from the city of Ancyra (modern Ankara, Turkey). He was a prominent figure in the intense theological debates that followed the First Council of Nicaea in 325 CE. Marcellus was a staunch defender of the Nicene Creed and the unity of God, positioning himself against theological opponents often called Arians or Eusebians.

His rigorous defense of divine unity led to significant controversy. His opponents accused him of Sabellianism or Modalism, a belief that the Father, Son, and Holy Spirit are not distinct persons but different modes of a single divine being. For this, he was deposed as bishop by a council in 335 and sent into exile. He later regained his position around 346, but his teachings remained suspect. Long after his death, the Council of Constantinople in 381 formally condemned his theology.

None of Marcellus’s own writings survive intact. His work is known only through fragments quoted by his critics, especially the historian Eusebius of Caesarea. His significant works include a treatise On the Holy Church and a major refutation Against Asterius the Sophist.

Modern scholars see Marcellus as a pivotal, if controversial, figure in the development of Christian doctrine. His radical emphasis on divine unity forced other theologians to refine their language about the Trinity, helping to shape the orthodox formulas that eventually distinguished between unity of essence and distinction of persons. While he was condemned as a heretic by the established church, his ideas played a key role in the complex process of defining Nicene orthodoxy.