Why even mention that "the dead in Christ will rise first" if it were an event that no one would know had happened? The resurrection of the dead in Christ just prior to His return to reign for a thousand years will be visible and known, just as the resurrected Christ was seen, touched, and witnessed eating food. Until the actual bodily resurrection of the dead in Christ takes place, the rapture of living saints cannot happen.
To tell us that it would happen, otherwise we wouldn't know.
If you read the whole passage, instead of just that verse, you'll see that Paul is reassuring the believers about the fate of their loved ones who had already died.