The long debate that preceded Mary's inclusion in Vatican II's Constitution on the Church, Lumen Gentium (lg) was won by a margin of only forty votes (1,114 in favour, 1074 against). The aim of such a decision was to put Mariological piety in its proper context. But such a compromise did not come without tensions: One such tension was the subtle clash of the 'Church as Mother' metaphor with the Church as the 'People of God'. Where the faithful are called to share in the priestly, prophetic, and kingly role of Christ as the People of God (lg Ch.2), a call to take active adult Church membership, the Mother Church metaphor encourages a dependent infant membership. This article investigates the use of the maternal ecclesial metaphor in lg and puts forward an Augustinian-Ambrosian sense of Mother Church that can support an ecclesiology which not only upholds Mary as ecclesia-type but also the Church as a People called to become like Mother Church themselves.