Many different themes can be found in the first act. In fact, the very first scene lays out most of the play's major themes. Listed below are a few:
The theme of men being honorable is first portrayed in this act. By the end of the play Shakespeare has shown us that the men who are thought to be the most honorable actually have significant flaws. However, the theme of questioning men's honorableness is first portrayed in the first scene via Beatrice's witty proclamations of her opinion of Benedick. The messenger claims that Benedick has performed well in the recent victorious wars and is a "good soldier" (I.i.44). Beatrice of course wittily questions his valor by saying that Benedick is a "good soldier to a lady; but what is he to a lord?" (45). Questioning Benedick's valor serves to question the valor of all the men who have been recognized as honorable.
A second theme we see portrayed in the first act is excessive pride. All throughout the play, Shakespeare shows us the consequences of excessive pride. Excessive pride is first seen in Beatrice's treatment of Benedick in the very first scene, especially her disdain for him. We know how much Beatrice disdains, or scorns him when we see him address her as, "What, my dear Lady Disdain! are you yet living?" (101).
The theme of appearances vs. reality is also first introduced in this first act. We learn from Leonato that Don John has just tried to overthrow Prince Don Pedro's thrown but has been forgiven and recently "reconciled" to the prince (132). For an Elizabethan, Leonato's line would raise a red flag. They would know from their own history that the only way to deal with treason is by execution and failing to do so would only bring more insurrection. Hence, Elizabethan's would know that not only is Don John untrustworthy, but the noble Prince Don Pedro's character should also be called into question as being not actually what it appears to be. They would know that this appearance of peace would soon show itself to be something else in reality very soon. Hence, Elizabethans would not have been surprised at all to see Don John speak ill of his brother later in this act and to plot against Claudio, whom he holds responsible for his own failure in overthrowing his brother's reign.