Cosmas Indicopleustes

When God said: In what day thou eatest of the tree, thou shalt surely die, he pronounced him to have become mortal, for we see that he did not die immediately according to the threat, so that it is evident the words hinted this: Immortal though thou art, thou shalt become mortal.

The Christian Topography, Book 5 ↗

Augustine of Hippo

God, referring to the forbidden fruit, said to the first man whom he had established in paradise: "In the day that you shall eat of it, you shall die the death." His threat included not only the first part of the first death, that is, the soul's deprivation of God; not only the second part of the first death, that is, the body's deprivation of the soul; not only the whole of the first death in which the soul, separated from both God and the body, is punished; but whatever of death is up to and including that absolutely final and …

City of God 13.12 ↗

Ambrose of Milan

What is the difference between saying 'ye shall die' and 'ye shall die the death'? We ought to point out that there is nothing superfluous in the command of God. Here is my solution. Since life and death are contradictory ideas, in unaffected language we say 'we live in life' and 'die in death.' But, if you wish, since life causes life, to double the force of the two concepts, the phrase 'he lives a life' is found in legal documents, and, since death causes death, there is the statement: 'He shall die the death.' [ Ezek 33:14-16 ] …

On Paradise ↗

Jerome

But on whatever day you eat from it, you shall surely die. Symmachus interpreted it better, saying, you shall be mortal.

Hebrew Questions on Genesis ↗