What doctrine was established at the Council of Nicaea?

The Council of Nicaea, convened in 325 AD, was primarily focused on addressing the Arian controversy, which revolved around the nature of Christ and his relationship to God the Father. The key outcome of this council was the formulation of the Nicene Creed, a statement that articulates the Christian belief in the Trinity, affirming that Jesus Christ is "true God from true God," and establishing his divinity as coequal with the Father. This creed served not only as a foundational statement of Christian orthodoxy but also as a unifying doctrine for Christians across the Roman Empire, countering various heretical views that had emerged.

The other options are significant in their own right, but they are unrelated to the Council of Nicaea. The Twelve Tables were an early Roman legal code; the Edict of Milan was a decree that established religious toleration for Christianity within the Roman Empire; and the Almagest is an astronomical text by Ptolemy that dealt with the geocentric model of the universe. None of these have a direct connection to the doctrinal discussions that took place during the Council of Nicaea.