Salus extra ecclesiam non est or Extra Ecclesiam nulla salus
There is no salvation outside the church.
On Baptism, Against the Donatists, book IV, ch. 17. Citing the famous teaching of St. Cyprian. In letter 185:50 (on the Donatist controversy), Augustine speaks of those who have knowingly separated from the unity of the Church: "Furthermore, the Catholic Church alone is the body of Christ, of which He is the Head and Saviour of His body. Outside this body the Holy Spirit giveth life to no one, seeing that, as the apostle says himself, 'The love of God is shed abroad in our hearts by the Holy Ghost which is given unto us;' but he is not a partaker of the divine love who is the enemy of unity. Therefore they have not the Holy Ghost who are outside the Church; for it is written of them, 'They separate themselves, being sensual, having not the Spirit.'" . Augustine does, however, allow certain exceptions, as for example, in cases of invincible ignorance. Eugène Portalié, S.J. writes: "God’s immediate influence on souls, however, is not hindered by this ordinarily indispensable role of the Church. That is an accusation of Protestants which Augustine had foreseen. (I) In the Church, God acts ceaselessly in souls through His graces as the interior teacher and inspirer of all good. (2) Outside of the Church, God’s hands are not tied: He can work marvels of grace without human intervention in souls who do not yet know the Church, as the case of the centurion Cornelius witnesses, who had received the Holy Spirit before being baptized. God acts thus to show more clearly that it is always He and not the minister who sanctifies: “Why does it happen now this way, now that way, unless to prevent us from attributing anything to our human pride but to divine grace and power?” The conclusion is that God sometimes sanctifies without the Church and the sacraments, but never one who scorns the sacraments: “Therefore we conclude that an invisible sanctification has been offered to some and used to advantage without visible sacraments.... Not on that account, however, is the visible sacrament to be scorned, for one who scorns it can in no way be sanctified invisibly.” God, History, and Dialectic: The Theological Foundations of the Two Europes (1997) by Joseph P. Farrell, Seven Councils Press, ISBN 0966086007 ISBN 9780966086003 p. 1013, also in A Guide to the Thought of St. Augustine (1960) by H. Regnery, pp. 232-233