Genesis 17:13 - Matthew Poole's English Annotations on the Holy Bible

So it was, and is properly in regard of the thing signified to all true believers; and for the sign, it is so called because it was to endure through all generations till the coming of the Messias; the word olam, here and elsewhere rendered everlasting, or for ever, being oft used to express not only simple eternity, but any long continuance, for many ages, yea, sometimes for a man's life. See Exodus 21:6 Deuteronomy 15:17 1 Kings 9:3.