Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 20

Chapter 20.—Of Those Who Promise This Indulgence Not to All, But Only to Those Who Have Been Baptized as Catholics, Though Afterwards They Have Broken Out into Many Crimes and Heresies.

There are others still who make this promise not even to all who have received the sacraments of the baptism of Christ and of His body, but only to the catholics, however badly they have lived.  For these have eaten the body of Christ, not only sacramentally but really, being incorporated in His body, as the apostle says, “We, being many, are one bread, one body;” 1534 so that, though they have afterwards lapsed into some heresy, or even into heathenism and idolatry, yet by virtue of this one thing, that they have received the baptism of Christ, and eaten the body of Christ, in the body of Christ, that is to say, in the catholic Church, they shall not die eternally, but at one time or other obtain eternal life; and all that wickedness of theirs shall not avail to make their punishment p. 468 eternal, but only proportionately long and severe.

Footnotes

1 Cor. 10.17.