Paul is giving us a command here, an imperative, but it actually goes further. In I Corinthians 16:13, there are four imperatives in this one verse, which is itself just six words in the original Greek: 1) watch, 2) stand fast in the faith, 3) be men (courageous), and 4) be strong.
The word watch means "to keep awake, be vigilant, be watchful." For us, that means keeping an eye on the world around us, and more importantly, paying attention to our spiritual condition. To stand fast in the faith means "to be stationary (anchored), to persevere, to be convicted of our beliefs." As we saw, to be men is "to be courageous," but not so much in a physical sense as in the convictions of our spiritual life. Finally, to be strong implies "to increase in vigor, to be strengthened, to increase in faith."
These four imperatives can be viewed in military terms, and Paul uses such terms quite often in his epistles. Living in the days of Roman rule, he commonly saw Roman legions in his travels. His audience, also living within the Empire, was quite familiar with soldiers and their duties.
We can imagine a sentry on guard duty, at attention, peering into the night, listening intently for any unusual noise. He has to fight off sleep lest the enemy sneak up on him and kill him, opening the camp to attack. We can realize how this applies to Christian life.
The other imperatives—standing fast in the faith, being strong, and living like men—are also better understood as military imagery. Many are familiar with the story of the Battle of Thermopylae in 480 BC, when Sparta's King Leonidas and his 300 Spartans, along with 700 Thespians, 400 Thebans, and perhaps a few hundred others, fought to the death against the million-man army of Persian Emperor Xerxes. King Leonidas and his men knew that they would die; they knew the odds were overwhelmingly against them. But they felt compelled to try to stop the enemy and save their country.
Certainly, this encapsulates these four imperatives!
As stunning as that example is, we should bring it down to a more personal level: to an army of one. Outnumbered as they were, the Spartans and their allies still had other warriors fighting with them on either side, at least until the very end. What if we were absolutely by ourselves?