Latin for 'The Peace of the Gods'.
This term is the essence of faith in Roman eyes. According to this perception of belief, Gods and Man have a treaty, according to which the Gods are due certain things, and in exchange for getting them they would not unleash their fury on mankind (or on specific individuals or groups within it), and might even, if they should so condescend, aid mankind occasionally. This was the essence of the Peace of the Gods.
In the eyes of the Romans the gods demanded prayers, rituals, sacrifices and, above all, respect. The gods did not expect you to like them, much less love them, nor did they expect piety from their believers (and indeed 'Pietas' in Classical Latin means more something like 'dutifulness' than spiritual devotion). For that reason the Romans put much stress on the external forms of worship, neglecting completely all form of spiritual revelation.
A public ceremony (and all ceremonies were that), for instance, that has been interrupted by anything from a mispronounced word to someone of the priests tripping, would be immediately stopped and conducted again from the beginning. This caused many ceremonies to lengthen to monstrous proportions, but this didn't matter, as the ceremony was the important issue, and not the intent.
Neglecting to perform your duties to the gods was a serious offence in Rome, punishable by death, because a personal violation of the Pax Deorum could bring the wrath of the Gods on the entire city.
Thus, the worship of the Roman Deities became thoroughly identified with the operations of the state, and one's duties to it. At the same time, however, a private person's involvement in the running and functioning of the state was decreasing; the Republic was dead, and the emperors didn't always even bother to pretend it was still there. Worship lost all its personal meaning and instead became a tedious and tiresome mandatory practice. Consequently, people began to find other sources of spiritual guidance. Oriental religions flourished in 1st and 2nd century Rome. Large cults were formed honouring Gods such as the Persian Mithras, the Egyptian Isis and Serapis (a typical syncretistic deity combining Zeus and Osiris) and Cybele from Asia Minor. This did not violate the Pax Deorum, as in polytheistic reality one could belive in as many gods as he chose to. The problem began with two other oriental cults: Judaism1 and then later Christianity.
People who converted to one of these religions could not participate in the Pax Deorum, as their faiths preached that the Roman Gods did not exist. Moreover these faiths declared that one should not take part in the ceremonies of those Gods, even if one didn't actually believe in what he was doing. This was a threat to the very existance and lives of the Roman city and its citizens. For this reason the authorities of the city and the Empire forbade preaching either one of these religions within the city, but as the preaching didn't stop, it lead to decleration of war against those religions. The attempts of the emperor Hadrian to abolish Judaism brought about three Jewish revolts, after the oppression of which Jerusalem, and much of the other cities of Judaea were destroyed, as was the existance of the Jews as an autonomous nation. Jews were scattered and deported from Judaea (the name of which was now changed to 'Palaestina'). Persecutions of Christians continued until the reign of Emperor Constantine.
I would like to thank
Cletus the Foetus for his aide in the formulation of the above.
1 There was actually one more problem with Judaism. You see, Roman law forbade a citizen from hurting, or damaging his body, this included tattoos, piercings and, yes, circumcision...