Marcellus of Ancyra (? - c. 374 C.E.) was one of the bishops present at the Councils of Ancyra and of Nicaea. He was a strong opponent of Arianism, but was accused of adopting the opposite extreme of modified Sabellianism. He was condemned by a council of his enemies and expelled from his see, though he was able to return there to live quietly with a small congregation in the last years of his life.
A few years after the Council of Nicaea (in 325) Marcellus wrote a book against Asterius, a prominent figure in the party which supported Arius. In this work (only fragments of which survive), he was accused of maintaining that the Trinity of persons in the Godhead was but a transitory dispensation. According to the surviving fragments, God was originally only One Being (hypostasis), but at the creation of the universe the Word or Logos went out from the Father and was God's Activity in the world. This Logos became incarnate in Christ and was thus constituted Image of God. The Holy Ghost likewise went forth as third Divine Personality from the Father and from Christ according to John 20:22. At the consummation of all things, however (I Corinthians 20:28), Christ will return to the Father and the Godhead be again an absolute Unity. The fragmentary nature of his surviving work makes reconstructing his thought more of an art than a science.
The bishops at the First Synod of Tyre in 335 (which also deposed Athanasius) seem to have written to Constantine against Marcellus when he refused to communicate with Arius at Constantine's thirtieth-anniversary celebrations at Jerusalem. Marcellus was deposed at Constantinople in 336 at a council under the presidency of Eusebius of Nicomedia, the Arian, and Basil of Ancyra appointed to his see. Marcellus sought redress at Rome from Pope Julius I, who wrote to the bishops who had deposed Marcellus, arguing that Marcellus was innocent of the charges brought against him. The Council of Serdica (343) formally examined his book and declared it free of heresy. But he seems not to have been reinstated in his see when Constantius II, threatened by his brother with war, allowed the restoration of Athanasius, and Paul of Constantinople to their sees in 345.
Athanasius' relations with Marcellus were complex, and communion between them was broken off for a time, but at the end of both their lives, Athanasius resisted Basil of Caesarea's attempts to have him generally condemned, and re-established communion with Marcellus. The Second Ecumenical Council condemned 'Marcellians', but not Marcellus himself. Eusebius of Caesarea wrote against him two works: "Contra Marcellum", possibly the prosecution document at Marcellus' trial, and "On the Theology of the Church" or "Ecclesiastical Theology", a refutation of Marcellus' theology from the perspective of Arian orthodoxy .
Marcellus's theology included a belief in universalism, that all people would eventually be saved. He is quoted by Eusebius as having said "For what else do the words mean, 'until the times of the restitution' (Acts 3:21), but that the apostle designed to point out that time in which all things partake of that perfect restoration."
Aside from the fragments which survive in Eusebius' Against Marcellus, a letter survives in Epiphanius, Panarion 72.