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Established the uniform Christian doctrine with the Nicene Creed; addressed the Arian controversy by confirming the Trinity, leading to the widespread acceptance of these beliefs in Christianity.

Counter-Reformation council that clarified Catholic doctrine and reformed clerical training and discipline; issued decrees on self-reform and defined the canon of the biblical books.

Reaffirmed the Nicene Creed and expanded the doctrine of the Holy Spirit; its additions to the Creed are still recited in Christian liturgy today.

Defined Christ as being in two natures, fully human and fully divine, which became a central tenet of Christian orthodoxy; caused a lasting schism with Oriental Orthodox Churches who rejected the definition.

Modernized the Catholic Church practices; allowed the use of the vernacular in Mass, emphasized the universal call to holiness, and encouraged the renewal of consecrated life and the importance of laypeople in the Church.

Affirmed the Virgin Mary as Theotokos (Mother of God); it challenged Nestorianism and affirmed the hypostatic union of divine and human natures in one person, Jesus Christ.

Disbanded the Knights Templar; made provisions for the prosecution of heretics and issued decrees for ecclesiastical reforms including the establishment of chairs for Oriental languages in universities.

Harmonized the calculation of the date of Easter and the style of tonsure throughout the English Church; it important for the Romanization of the Church in England.

Reestablished the veneration of icons and images in the Christian Church; formally ended the first period of Byzantine iconoclasm and had lasting impact on Christian art and liturgy.