This phrase is the tag line of our production, and recently I've been thinking about it quite a bit in relation to other aspects of character of Macbeth and the show at large.
The phrase is spoken by the witches as Mac enters their Pit of Acheron for his second visitation, to learn by the worst means, the worst.
Most obviously the phrase resonates the porter's speech about the knocking at hell's gate.
But yet another textual association can be drawn to Mac's speech in I.iii, when he he is rapt in thought in regards to his new robes. "This supernatural solicitation can not be ill, cannot be good, if ill why hath it given me earnest of success commensing in a truth; if good, why do I yield to that suggestion whose horrid image doth unfix my hair, and my seated heart KNOCK at my ribs, against the use of nature."
By such ligamenting, it can be said, his very heart, moved by the witches' prohecy, is it self knocking at hell's gates, his ribs, if one can see the ribs as two reciprocal panels of hinged barred doors, like that of some castle entrance. And even the smallest knocking of sin, at the perpetrator's heart is enough to summon the porter to his calling.