So from what time has this so simple nature, the Creator and Sustainer* of all things, existed? And until what time will he continue to exist? Or does he instead exist, not from some time and until some time, but rather without beginning and without end? For if he has a beginning, either he has it from himself or through himself, or he has it from another or through another, or he he has it from nothing or through nothing. But through the truth that has already been discerned, it has been established  that he in no way exists from another or from nothing, or through another or through nothing. So no beginning was in any way allotted to him through another or from another, or through nothing or from nothing.
But he cannot have a beginning from himself or through himself, although he does exist from himself and through himself. For he exists from himself and through himself in such a way that the essence he is through himself and from himself is in no way different from the essence through which and from which he exists. By contrast, whatever begins to exist from something or through something is not in every respect the same as that from which or through which it begins to exist. Therefore, the supreme nature did not begin to exist through himself or from himself. Therefore, since he has no beginning through himself or from himself, or through another or from another, or through nothing or from nothing, he in no way has a beginning.
But neither will he have an end. For if he is going to have an end, he is not supremely immortal and supremely incorruptible. But it has been established that he is supremely both immortal and incorruptible. Therefore, he will not have an end. Moreover, if he is going to have an end, he will perish either willingly or unwillingly. Now surely that by whose will the supreme good perishes is not an unqualified* good. But he himself is a true and unqualified good. Therefore, he who assuredly is the supreme good will not perish voluntarily. But if, on the other hand, he is going to perish unwillingly, then he is not supremely powerful or all-powerful. But the necessity of reason has declared that he is supremely powerful and all-powerful. Therefore, he will not perish unwillingly. And so,if the supreme nature will not have an end neither willingly or unwillingly, he will in no way have an end.
St. Anselm, Monologion, trans. Thomas Williams (Hackett: 1995) p. 14
-  Anselm discusses this point in chapter 3, 5, and 6 of his Monologion.