Abraham is the state of consciousness that holds to a promise it cannot yet see, and Isaac, the child promised, means he laughs, the resurrection that seems too absurd to believe. The laughter is right there in the name, because the pledge of new birth out of a worn-out life sounds ridiculous to reason, and reason is exactly what laughs.
The hundred years mark the back of the head, the very place where the son is born from above, so the number is not old age but the birthplace of the awakening within a man. Sarah is the one from above who bears freedom, not slavery. The child comes by grace, not by effort, and no one is finally left out of it. In plain words, the new life promised to you feels impossible only to the calculating mind, and it arrives anyway, born in you when its own hour comes.