The "Equivocator" at the gate

The drunken Porter responds to the knocking at the castle's gates just after Macbeth has murdered King Duncan. As he does so, he imagines there's a Catholic "equivocator" at the door "who committed treason enough for God's sake" (2.3.1). This is almost certainly a reference to Jesuit Henry Garnet, a man who was tried and executed for his role in the Gunpowder Plot of 1605 (an unsuccessful attempt by a group of Catholic extremists to blow up Parliament and King James I with a keg of gunpowder). Henry Garnet wrote the "Treatise on Equivocation," which encouraged Catholics to speak ambiguously or, "equivocate" when they were being questioned by Protestant inquisitors (so they wouldn't be persecuted for their religious beliefs). This is exactly what Garnet did when he stood trial for treason.

Equivocation (speaking ambiguously or not telling the whole truth) resurfaces throughout the play. The witches tell partial truths when they make predictions, Macbeth frequently bends the truth as he deliberates about whether or not it's OK to murder the king, and Macbeth also equivocates when he justifies (to his henchmen) that murdering Banquo is acceptable.