metaph., the military sacramentum The solemn oath taken by the conscript on his enrolment. It bound the soldier to the state and his superiors and rendered accursed anyone who broke it.
Sacramentum - The Encyclopedia of Ancient History - Phang - Wiley Online Library
Interesting that to be a soldier of Christ the oath of allegience changes your life from that of a worldly citizen into a citizen of the Kingdom by the grace of God and to carry out His commands, whatever they might be, perhaps as a reaper for the harvest?
Psa 60:4 Thou hast given a banner to them that fear thee, that it may be displayed because of the truth. Selah.
Psa 13:2 Lift ye up a banner upon the high mountain, exalt the voice unto them, shake the hand, that they may go into the gates of the nobles.
3 I have commanded my sanctified ones, I have also called my mighty ones for mine anger, even them that rejoice in my highness.
4 The noise of a multitude in the mountains, like as of a great people; a tumultuous noise of the kingdoms of nations gathered together: the LORD of hosts mustereth the host of the battle.
6 Howl ye; for the day of the LORD is at hand; it shall come as a destruction from the Almighty.
To perform their transformation from Roman citizens into Roman soldiers, the selected men would then have to swear an oath of allegiance.
This swearing of the sacramentum, changed the status of the man entirely. He was now utterly subject to his general's authority, and had thereby laid down any restraints of his former civilian life. His actions would be by the will of the general. He would bear no responsibility for the actions he would commit for the general. If he was ordered to do so, he would kill anything in sight, be it an animal, a barbarian, or even a Roman.
There was more than mere practicality behind the change from the white toga of the citizen to the blood red tunic of the legionary. The symbolism was such that the blood of the vanquished would not stain him. He was now no longer a citizen whose conscience would not allow for murder. Now he was a soldier. The legionary could only be released from the sacramentum by two things; death or demobilization. Without the sacramentum, however, the Roman could not be a soldier. It was unthinkable.
Becoming a Soldier