Knock, knock! Who's there, in the other devil's
name? Faith, here's an equivocator, that could
swear in both the scales against either scale;
who committed treason enough for God's sake,
yet could not equivocate to heaven: O, come in, equivocator.
Many will tell you that the Porter scene is comic relief, but they are wrong. His language and actions may seem comical by today's audience, but to Elizabethans (at least the educated ones), his performance is grotesque, and his language is not unlike the devil's.
First of all, the most heinous crime in all of Elizabethan England or in any Medieval honor culture is to kill a king. Regicide is akin to killing God. Duncan's murder is not shown on stage for a reason.
Next, the knocking on the door parallels the sound imagery from the scene before, with the bell. Macduff is the hero of the play. He was born not of woman (like virgin birth). He sacrifices his family for the salvation of the country. He is a Christ-figure, and his knocking is a foreshadowing of the salvation to come.
I think the Porter's speech is grotesque, rather than comic. Some critics think he's the devil. All would agree that Macbeth's castle has become a hell, so who else would guard its gates?
The Porter also talks of equivocations (half-truths, paradoxes), like the witches. The Devil is the great equivocator: he seduces with lies and false promises. He tempted Christ with "Man shall not live with bread alone" and Eve with "You shall not surely die."
Equivocal morality frightened Elizabethans, almost as much as regicide. To blur the lines of good and evil was a great fear. We have all seen what moral relativism has done to the power of the church and the slackening of traditional moral values. It was indeed a death knell to Christian theology and opened the door of the occult philosophy that was prevalent in the day.
The "porter scene" in Act II Sc.3 is significant for the following reasons:
1. Comic Relief: the porter's speech is ribald and would have appealed to the coarse nature of the "groundlings," especially these lines which refer to the after effects of the consumption of excess alcohol:
Marry, sir, nose-painting, sleep, and
urine. Lechery, sir, it provokes, and unprovokes;
it provokes the desire, but it takes
away the performance: therefore, much drink
may be said to be an equivocator with lechery:
it makes him, and it mars him; it sets
him on, and it takes him off; it persuades him,
and disheartens him; makes him stand to, and
not stand to; in conclusion, equivocates him
in a sleep, and, giving him the lie, leaves him.
This scene immediately follows an intensely tragic scene in which, according to the medieval "Divine Right" of Kings, God's own representative on earth has been murdered by Macbeth. For the contemporary Elizabethan audience, Macbeth's treacherous act almost amounts to killing God himself. The Porter Scene which follows immediately provides the much needed relief to the emotions and the feelings of the audience.
2. Ironic contrast: Macbeth's castle has indeed become "hell" because Macbeth has murdered Duncan the King of Scotland, God's own representative on earth. The comic scene serves as a foil to the tragic scene which precedes it and thus, increases its intensity.
3. Prosodic contrast: the Porter's speech is in prose in keeping with his low social status, unlike the rest of the play which is in blank verse.
4. Social satire: The social satire behind the references to the "farmer," the "tailor" and the "equivocator" would have been immediately understood by Shakespeare's contemporary audience.