The Third Session of Vatican II (Sept-Nov 1964) focused on major Church documents like Lumen Gentium (Dogmatic Constitution on the Church, defining it as the People of God) and Sacrosanctum Concilium (Constitution on the Sacred Liturgy, allowing vernacular), alongside debates on religious freedom and ecumenism, culminating in decrees on bishops' roles (Christus Dominus) and priestly training, setting foundational changes for lay involvement, bishops' collegiality, and modern engagement, though controversies like birth control were deferred. The Third Session solidified Vatican II push for aggiornamento (bringing up to date), shifting the Church's self-understanding towards communion and mission, and initiating significant reforms in liturgy, Church structure, and its relationship with the modern world and other faiths.