Ecumenical (universal) councils are solemn assemblies of all the world's bishops, convened by or with the approval of the Pope, to define doctrine, condemn heresies, and reform Church discipline. The Catholic Church recognizes 21 ecumenical councils, beginning with the Council of Nicaea I in 325 AD and most recently the Second Vatican Council (1962-1965). Some of the most significant include: Nicaea I (325) — defined the divinity of Christ and produced the Nicene Creed. Constantinople I (381) — affirmed the divinity of the Holy Spirit. Ephesus (431) — declared Mary as Theotokos (Mother of God). Chalcedon (451) — defined that Christ has two natures (divine and human) in one person. Trent (1545-1563) — responded to the Protestant Reformation, clarified Catholic doctrine on justification, sacraments, and Scripture. Vatican I (1869-1870) — defined papal infallibility. Vatican II (1962-1965) — renewed the Church's engagement with the modern world. The decisions of ecumenical councils, when confirmed by the Pope, are considered infallible and binding on all Catholics.