Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 12

Chapter 12.—What Death God Intended, When He Threatened Our First Parents with Death If They Should Disobey His Commandment.

When, therefore, it is asked what death it was with which God threatened our first parents if they should transgress the commandment they had received from Him, and should fail to preserve their obedience,—whether it was the death of soul, or of body, or of the whole man, or that which is called second death,—we must answer, It is all.  For the first consists of two; the second is the complete death, which consists of all.  For, as the whole earth consists of many lands, and the Church universal of many churches, so death universal consists of all deaths.  The first consists of two, one of the body, and another of the soul.  So that the first death is a death of the whole man, since the soul without God and without the body suffers punishment for a time; but p. 251 the second is when the soul, without God but with the body, suffers punishment everlasting.  When, therefore, God said to that first man whom he had placed in Paradise, referring to the forbidden fruit, “In the day that thou eatest thereof thou shalt surely die,” 593 that threatening included not only the first part of the first death, by which the soul is deprived of God; nor only the subsequent part of the first death, by which the body is deprived of the soul; nor only the whole first death itself, by which the soul is punished in separation from God and from the body;—but it includes whatever of death there is, even to that final death which is called second, and to which none is subsequent.

Footnotes

Gen. 2.17.