A Libellus or Certificate acted as official confirmation that the person named on it had obeyed the imperial command and had worshipped the Roman emperor. Many hundreds of these have been found. They were written on a piece of parchment by the person offering the sacrifice and signed by the official.
This example was from during the reign of Emperor Decius, around AD 250. This was one of the periods in Roman history that worshipping the emperor was made compulsory, with severe punishments for failing to do so. Decius made an edict commanding provincial governors and magistrates to supervise the sacrifices to the gods and to the spirit of the emperor. All citizens were required to do this.
To:THE COMMISSIONERS FOR SACRIFICE IN THE VILLAGE OF ALEXANDER'S ISLAND
SON OF SATABUS
OF THE VILLAGE OF ALEXANDER'S ISLAND
SCAR ON RIGHT EYEBROW
I have always sacrificed to the gods, and now in your presence, in accordance to the terms of the edict, I have done sacrifice and poured libations and tasted the sacrifices, and I request you to certify to this effect. Farewell.
Presented by me: AURELIUS DIOGENES
I CERTIFY THAT I WITNESSED HIS SACRIFICE:Aurelius Syrus
This first year of the Emperor Caesar Gaius Messius Quintus Trajanus Decius, Pius, Felix, Augustus, the 2nd of Epiph (26th June 250)