In ancient rome, the symbol of being a free roman citizen was the pileus hat. Paul was telling them to take it off (which was shocking at the time, because they wore them everywhere, at work, during dinner, in the shower. Yay we are FREE!) So in taking off the symbol of there freedom, and in turn, looking like a slave, they were showing themselves to be slaves of Christ.
So we still take our hat off in church in respect to what was said to the 1st century church.
Well that's interesting. I've never heard that before! (The Roman citizen hat explanation in connection to this passage.)
There were other ways (for certain populations) that one could prove that either they weren't a slave, or that if they were; they belonged to a certain household.
In the military; they'd be tattooed with their legion or commander's symbol. People of all sorts of ethnicities could be in the army. And the "mark" that indicated "military" was actually a "cross". (It was a Greek letter that was no longer in use any more, that was called "a cross"; although what it technically looked like, I don't know. It most probably resembled a + sign.) And next to this would be the legion number or commander's symbol (or maybe even both).
Captives taken in combat were considered slaves; but one could become a citizen by joining the army. They'd have to have permission by the slave owner to join the army; although "obedient combat captives" would be owned by military people. After 20 years a soldier earned citizenship. Which citizenship was a BIG DEAL in ancient Rome. They also tattooed free born men who joined the army, that they couldn't be sold as slaves.
And there likely was a similar civilian accommodation for tattooing slaves whom both master and slave wanted to remain together.
Actually getting into the Roman army was an ordeal in and of itself. If one made it all the way through training; they were assigned a legion. (For a fully trained recruit assigned a legion; I believe their "basic training" was about 6 months. And on top of that; a fully trained legionnaire had been in the army about 2 years. With a few exceptions - like those who were assigned to be executioners - soldiers were trained in every combat position.) Once you were officially in the army; you'd joined for life. Your legion became your family. And a lot of military people actually had family members who were also in the military.
The military was the only way people who didn't have money could become citizens and earn "ranking" in Roman society. Obviously the army had a ranking system and the higher up in rank you went; the higher prestige you had in the eyes of civilian citizens. Soldiers who were over 40 years old were assigned to things like guarding prisons. They could also be "territorial proctors". Similar to like a combination of town mayor and police chief.
Pontius Pilate would have been in the army and he was probably over 50 years old at the point he was a proctorate in Judea. Civilians could be proctors too; but they would not have chosen to be proctors over Roman territories. (too much potential inherent conflict) So someplace like Judea (because of revolts against Rome) would have only been taken by a military person.
People who made good rank in the Roman army had a high degree of personal discipline. Those who were the most "pragmatically even tempered" would be offered position of executioner. (Those that carried out crucifixions and / executing soldiers. These were well paid positions.) Army executioners also dealt with citizens who were given a death sentence. Citizens could be executed after a formal trial. Citizens were beheaded. Also, Rome did not crucify the insane, females or children. Common crimes citizens were executed for were murder, treason and rape (against female citizens).
One "benefit" of being in the army is that it was illegal to crucify soldiers. It was considered too much of a shame. Soldiers who'd committed crimes worthy of death were killed by the men in their own legion. That was considered a "redeemable act" to face one's own death as a soldier and not a coward.
Executioners would use an "execution flagellum" on soldiers. This was the type of whipping implement that would have pieces of glass and metal in it designed to rip the skin and muscles. The execution flagellum was designed that the soldier would bleed out and die long before the designated "40 lashes" was complete. This type of flagellum was not used on someone who was to be crucified; because the intent of crucifixion was that the death be a prolonged as possible. Also the execution of soldiers was only "public" to the legion members. Of which, post death they'd be given a soldier's burial.
So thus when Jesus was flogged; they used a "chastisement flagellum". We know this because Pilate stated that he would "chastise" Jesus and let him go. (Luke 23:16) "Chastisements" weren't meant to be lethal; just unpleasant. The army used the same system on disobedient soldiers who did not commit crimes considered worthy of death. Soldiers would be "chastised" for things such as petty theft or brawling among the ranks. Which like OT law; a "chastisement" in the army was 39 lashes with a 3 strand leather whip; which would have constituted 13 strikes. (39 lashes) So no, Jesus would not have been struck with an "execution flagellum" because that was a death reserved for soldiers.