As Augustus returned to Rome in 13 BC, the Senate passed a constitutio to build in his honor a lasting altar of peace, the Ara Pacis Augustae, to signal with a major ceremony the new peace all over the Roman world, Gibbon’s Pax Romana. As we know from Ovid Fast. 1. 709–714, 3. 881–882, the Ara Pacis was the site of two annual sacrifices (on 30 Jan. and 30 March) to Pax, an innovation of the Augustan Age, for formerly Pax had been a minor goddess without a temple. The Augustan regime elevated a new form of Pax as a religious cult and made it acceptable to the Roman people, who had regarded Pax as the phenomenon of a foreign power too beaten down to resist Roman arms any longer and had no use for pacifism (in the modern sense), which would be seen only as cowardly in their dangerous world.
Augustus had started this process, perhaps not intentionally, back when he closed the Gates of Janus in 29. By bringing together Greco-Roman elements of Pax with Jupiter and Janus, he was able to forge a new religious cult to Pax Augusta that could appeal to the average Roman by its promise of prosperity and the absence of civil war. Foreign war was perfectly acceptable and not incompatible with this cult, but the emphasis was on domestic harmony and old traditional religious practices, even if the average listener could not understand some of these obligatory, archaic chants. For this reason, the third closing of the Gates of Janus very likely accompanied one of the Ara Pacis ceremonies.
Augustus also built on precedents from his divine father Julius, who had founded the towns Forum Iulii Pacatum (Fréjus, France) and Pax Iulia (Beja, Portugal) and issued Pax imagery on coinage to gain the moral high ground during the civil war. Augustus went one step further with larger sets of Pax coin issues to tell the people that he, not Antony, was trying to maintain peace when Cleopatra wanted war, and then a sequel after Actium that demonstrated his ability to prevail and restore order. The image of Pax Augusta evolved as it developed, but the epitome is the goddess we see on the East side of the Ara Pacis, surrounded by fertility and prosperity, in a state of security. Rome too would enjoy the same benefits.