Athanasios of Alexandria was a prominent Christian theologian and bishop of Alexandria in the 4th century. He is best known for his strong opposition to the Arian heresy, which denied the divinity of Jesus Christ. Athanasios played a crucial role in the development of early Christian doctrine, particularly at the Council of Nicaea in 325 AD, where the Nicene Creed was established. He served as bishop for 45 years, with several periods of exile due to his conflicts with Arian leaders and political authorities. Athanasios is also recognized for his writings, including the influential work "On the Incarnation," which articulates the significance of Christ's divine nature and humanity.