Marcellus of Ancyra was a Christian bishop in the early 4th century, known for his opposition to Arianism, a belief that denied the full divinity of Jesus Christ. He played a significant role in the theological debates of his time, particularly during the Council of Nicaea in 325 AD, which sought to address the controversies surrounding Arianism. As a bishop, Marcellus emphasized the unity of the Father and the Son, arguing against the teachings of Arius. His writings and teachings contributed to the development of early Christian doctrine, and he is remembered for his staunch defense of the Nicene faith.