Isidore, Sententiae 1.1.2

In Sententiae 1.1.2, Isidore equates God’s immortality with his immutability:¹

2. Quid est Dei immortalitas, nisi eius incommutabilitas? Nam et angeli et animae immortales sunt, sed immutabiles non sunt; ideoque solus Deus dicitur immortalis, quia solus incommutabilis est. Nam anima moritur, dum, deserente Deo, de bono in malum mutatur, sic et angelus, dum, deserente Deo, est lapsus.

2. What is the immortality of God, except his immutability? For both angels and souls are immortal, but they are not immutable; and therefore God alone is said to be immortal, because he alone is immutable. For the soul dies when, with God abandoning it, it is changed from the good into evil; so also an angel when, with God abandoning it, enters into a state of fallenness.

What Isidore does here is to distinguish between God’s “immortality” and the “immortality” of creatures–a distinction reminiscent of and not unrelated to, though also not identical with, Boethius’ in Book 5 of the Consolation, as well as in his treatise on the Trinity, between “sempiternity,” everlastingness or constant existence through the progression of time, and “eternity,” the timeless existence possessed by God alone (cf. here). That is to say, there is a qualitative difference between God and creation (we observed that the same distinction was significantly operative in the first installment as well). Angels and human souls can be said to be immortal, but only by bestowal from God. If he removes his sustaining hand, they fall into death; they do not possess immortality intrinsically, and so they are changeable. God, however, is immortal in a different way: he is unchangeably immortal, and thus his immortality can be identified with his immutability.