What heresy did the Council of Nicaea condemn?

The Council of Nicaea, convened in 325 AD, is primarily known for addressing the theological dispute surrounding Arianism. This heresy, proposed by Arius, asserted that the Son of God was created by the Father and therefore was not co-eternal or of the same substance as the Father. This challenged the very nature of Christ and his divinity, which was critical to Christian doctrine.

The Council's response was to affirm the Nicene Creed, which established the belief in the consubstantiality of the Son with the Father, meaning that both are of the same essence. This was a significant moment in the development of Christian theology, as it laid the groundwork for future Christological discussions and helped to unify the Church against Arian teachings.

In contrast, the other options represent different theological disputes. Pelagianism revolved around the nature of sin and grace, Modalism dealt with the understanding of the Trinity, and Donatism was concerned with the validity of sacraments administered by clergy who had renounced their faith. Although these heresies were significant in their own right, they were not the primary focus of the Council of Nicaea, which was specifically aimed at resolving the controversy caused by Arianism.