Necessity distinguished from immutability and eternity
But he errs who thinks this attribute ought to be reduced either to immutability or to eternity. For just as for things to be changed properly is not for them either to be produced so that they are, or to be destroyed so that they are not, but with existence remaining safe to acquire or lose something formally — so conversely to be immutable is not the same as not to be able not to be; but properly signifies not to be able to acquire a new perfection, or to be deprived of any, with existence remaining safe.
From which it is gathered that necessity of existing and immutability differ not a little; since this is opposed to mutability, that to contingency. But eternity is nearer to this necessity, although they differ in this, that this denies the beginning and end of duration, but that denies contingency or the power of not existing: and therefore, as immutability is a certain cause of eternity, of which it is the prior cause and reason, so this necessity is a cause of eternity. And so we conclude that God is eternal — that is, lacks beginning and end of His duration — from this, that He Himself is a necessary being, to whom not to be is altogether adverse.
Latin
Errat vero, qui putat, hoc attributum, vel ad immutabilitatem, vel ad aeternitatem revocari debere. Quemadmodum enim res mutari proprie non est eas vel produci, ut sint, vel destrui, ut non sint: sed salva exsistentia aliquid formaliter acquirere vel amittere. Ita contra immutabilem esse, non est idem atque non posse non esse; sed proprie significat, non posse novam perfectionem acquirere, vel aliqua privari, salva manente exsistentia. Ex quibus colligitur, non parum differre necessitatem exsistendi & immutabilitatem; cum haec mutabilitati, illa contingentiae opponatur. Est autem huic necessitati aeternitas propinquior, etsi in hoc differant, quod haec durationis principium & finem, illa vero contingentiam sive potentiam non exsistendi negat: ideoque, ut immutabilitas aeternitatis quaedam, qua causa est & ratio prior, ita haec necessitas aeternitatis. Atque ita, Deum aeternum esse, hoc est, durationis suae principio & fine carere, conficimus ex hoc, quod ipse est ens necessarium, cui non esse omnino adversatur.