In Act III, Scene 2, what does Hermione's speech mean, "Since what I am to say must be but that"?

This is Hermione's speech at her trial for adultery.  In the first several lines (3.2.22-28), she says that because it is her word against Leontes', then there is no point in saying "Not guilty."  Since Leontes' jealousy has made any sort of defense impossible, Hermione can only rely on a higher power.  She bravely and beautifully declares: "if powers divine / Behold our human actions, as they do, / I doubt not then but innocence shall make / False accusation blush" (3.2.28-31).  She concludes by reminding Leontes' that he cannot deny her conduct was entirely honorable before Polixenes came to the court and this unhappy situation started.  This reliance on "powers divine" prepares for the revelation of the oracle later in the scene, which in turn leads to the conclusion of the tragic first half of the play.