In "Macbeth", how do the Porter's lines link with the themes of the play?
Well, the idea of a porter of "hell-gate" (someone portering the doorway into hell) is appropriate for a play very concerned with evil spirits, with witches, and which leaves Macbeth, in his final scenes, calling out for a character named "Seyton" (which sounds rather a lot like "Satan" on the stage).
More than that, though, the porter's lines reflect some of the other concerns of the play. Obviously the idea of hell and damnation, coming straight after the murder scene, resonates with the fates of the Macbeths.
But more central than that is the concept of equivocation:
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.
The concept of equivocation - "swearing in both the scales against either scales" - is important. Macbeth describes the witches as "juggling fiends", and indeed, their language, like much of Macbeth's language seems unusually equivocal - almost to the point of absurdity. Foul is fair and fair is foul. Nothing is but what is not. Macbeth will be king, but Banquo's sons will be kings. It's difficult to pull out one logical meaning for many of the things that are said in Macbeth: rather like in algebra, it becomes a process quite like adding two negatives to get a positive. The language itself, like the air-drawn dagger, equivocates. It both is, and is not.