Works of St. Anselm, tr. by Sidney Norton Deane, [1903], at sacred-texts.com
It is without beginning and without end.
FROM what time, then, as this so simple Nature which creates and animates all things existed, or until what time is it to exist? Or rather, let us ask neither from what time, nor to what time, it exists; but is it without beginning and without end? For, if it has a beginning, it has this either from or through itself, or from or through another, or from or through nothing.
But it is certain, according to truths already made plain, that in no wise does it derive existence from another, or from nothing; or exist through another, or through nothing. In no wise, therefore, has it had inception through or from another, or through or from nothing.
Moreover, it cannot have inception from or through itself, although it exists from and through itself. For it so exists from and through itself, that by no means is there one essence which exists from and through itself, and another through which, and from which, it exists. But, whatever begins to exist from or through something, is by no means identical with that from or through which it begins to exist. Therefore, the supreme Nature does not begin through or from, itself.
Seeing, then, that it has a beginning neither through nor from itself, and neither through nor from nothing, it assuredly has no beginning at all. But neither will it have an end. For, if it is to have end, it is not supremely immortal and supremely incorruptiblep. 68 . But we have proved that it is supremely immortal and supremely incorruptible. Therefore, it will not have an end.
Furthermore, if it is to have an end, it will perish either willingly or against its will. But certainly that is not a simple, unmixed good, at whose will the supreme good perishes. But this Being is itself the true and simple, unmixed good. Therefore, that very Being, which is certainly the supreme good, will not die of its own will. If, however, it is to perish against its will, it is not supremely powerful, or all‑powerful. But cogent reasoning has asserted it to be powerful and all‑powerful. Therefore, it will not die against its will. Hence, if neither with nor against its will the supreme Nature is to have an end, in no way will it have an end.
Again, if the supreme nature has an end or a beginning, it is not true eternity, which it has been irrefutably proved to be above.
Then, let him who can conceive of a time when this began to be true, or when it was not true, namely, that something was destined to be; or when this shall cease to be true, and shall not be true, namely, that something has existed. But, if neither of these suppositions is conceivable, and both these facts cannot exist without truth, it is impossible even to conceive that truth has either beginning or end. And then, if truth had a beginning, or shall have an end; before it began it was true that truth did not exist, and after it shall be ended it will be true that truth will not exist. Yet, anything that is true cannot exist without truth. Therefore, truth existed before truth existed, and truth will exist after truth shall be ended, which is a most contradictory conclusion. Whether, then, p. 69 truth is said to have, or understood not to have, beginning or end, it cannot be limited by any beginning or end. Hence, the same follows as regards the supreme Nature, since it is itself the supreme Truth.