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Why did the First Council of Constantinople in 381 condemn Arianism again?

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Christology The Transfiguration, the nature of Jesus as the Son of God being revealed to the apostles Peter, James, and John, mosaic icon, early 13th century; in the Louvre, Paris.

The First Council of Constantinople in 381 did not condemn Arianism "again" because the heresy had been fully eradicated; rather, it was necessary because Arianism remained a persistent and influential force within the empire long after the First Council of Nicaea in 325.1

Despite the initial condemnation at Nicaea, the controversy continued for decades as various factions—including those who argued the Son was "like" (homoios) or "of similar substance" (homoiousios) to the Father—gained imperial support and ecclesiastical influence. Throughout the 4th century, different councils issued conflicting statements of faith, and Arian-leaning emperors frequently suppressed the Nicene party.1

By the time Emperor Theodosius I convened the council in 381, the goal was to finally consolidate the non-Arian position and restore theological unity. The council formally proscribed Arianism and approved the Nicene Creed, which affirmed the full divinity of the Son and the Holy Spirit, thereby providing a definitive Trinitarian doctrine to counter the Arian view that Christ was a created being.12