Shakespeare uses Antonio's and Sebastian's comments to foreshadow and then demonstrate they are villainous characters who mock or lack virtue. Notably in Act 1 Prospero mentions that Miranda has the "fortitude of heaven"; fortitude is also a cardinal virtue.
The phrase was originally used in The Tempest, Act 2, Scene I. Antonio uses it to suggest that all that has happened before that time, the "past," has led Sebastian and himself to this opportunity to do what they are about to do: commit murder.
