If one takes the oath prior to the government's presentation of its case, how could one know whether or not the government would act lawfully?
All citizens have a duty to uphold the Constitution. Naturalized citizens and members of the military are bound by oath; other citizens are not bound by oath but still have such a duty.
Are you comfortable with 12 random people making that judgment? Do you think they know the Constitution well enough to say one way or the other?
On some issues, no. On some other issues, however, I would expect judges to be wrong most of the time, and jurors should serve as a backstop.
For example, the Fourth Amendment explicitly forbids unreasonable searches and seizures. To be legitimate, a search must be conducted in reasonable fashion. A typical judge, asked if a search is "reasonable", will look at whether anyone has managed to rationalize as reasonable any sort of search vaguely similar to the one at hand. An ordinary citizen, on the other hand, would be more likely to simply look at whether the search was in fact reasonable.
To be sure, there won't always be agreement about what's reasonable, but some cops engage in conduct which may vaguely resemble "reasonable" behavior but is nonetheless clearly unreasonable. Jurors, if allowed the chance, might correctly recognize that sometimes the cops are the robbers.
Where does the Constitution give you the authority to decide what "lawful" means?
On some issues, no. On some other issues, however, I would expect judges to be wrong most of the time, and jurors should serve as a backstop.
"Judges" wrong "most of the time", huh? Just any ol' random judge, out of the thousands of courts in the country, and you can say with a straight face that you would "expect [him] to be wrong most of the time." depending on the subject.
Wow. I thought Barack Obama was arrogant!