During the first century, the phrase ‘Jesus is Lord’ was used as a contrast to the popular greeting amongst Roman citizens who said ‘Caesar is Lord.’ Because early Jesus followers refused to swear allegiance to the Roman empire (and its wars and merchants and kings, Rev 12), the empire saw their refusal as a social, religious, and political threat. In the Roman world emperors encouraged an imperial cult following, proclaiming and deifying themselves Lord and 'sons of god' and they were not open to being challenged. Emperors viewed their rule as divinely authorized and protected and demanded that others view it the same way.
In first century social customs, honor was prized above almost all other things. Honor was measured in accolades from others, so a key part was that honor must be proclaimed publicly, or it wasn't honor. For Judean-Christians to refuse to call Caesar 'lord' caused two problems for the empire. First, it was a public insult to the honor of the emperor and Rome. Worse, in proclaiming Jesus Lord instead of Caesar, these Judean-Christians were saying that their God deserved more honor than Caesar who had destroyed Jerusalem and made huge numbers of them slaves and refugees. Second, it showed that the Judean-Christians believed that Caesar did not rule by the power of the supreme God of the Cosmos. In asserting 'Jesus is Lord' they were saying, instead, that Jesus (and YHWH) was the supreme God of the Cosmos and Caesar was not.
In the first century, religion and politics were mixed together: both Rome and the Judea/Kingdom of Judah were theocracies and ruled by officials who claimed divine power, protection, and patronage. To assert one’s religious views, then, would have had a subversive impact on the political system at that time. But many of these early Judean-Christians had no desire to be political in the way we know it today. It was dangerous to provoke Rome. What early Christians did to help others, and that Christians spread the word of God, was far more important than overthrowing the empire (God, they believed, would do that). The works of Paul, especially I Corinthians 11, entreats Christians, especially gentile Christians, to try to blend in and not make waves. Even Jesus himself, who clearly speaks against abuses of power, suggests making the payment of taxes exacted by Roman Emperors (Mk 12:17). He clearly does so, however, without granting Caesar any honor or esteem.
The Book of Revelation was an example of Judeo-Christian writings of the day that directly challenged the rule of the Caesars and their claim to divinity. The story tells us that the trouble was more than just conflicts between human armies and badly behaving kings. Revelation asserts that kingdoms and armies drew their power from cosmic forces. The great dragon depicted in Revelation represented the power of Chaos. God’s angels fight to defeat and contain that chaos in order to bring justice to heaven and earth.
John of Patmos, writer of Revelation, shows us that the evil was too big a problem for humans to solve alone. Rome was drawing power from Chaos, and only God could contain it. But that didn’t mean humans didn’t have a part to play. The saints (some of whom had been persecuted and martyred) and followers of Jesus clearly were important to the effort. Revelation 12:11-12 tells us that the hosts of heaven were victorious because of Christ, but also because of those who were witnesses of Christ, who spoke their experiences of the living God and the potential and hope for God’s people—and who did so even if it meant sacrificing their own lives. From this we know the importance of the saints in the battle waged in heaven and on earth as God moves to defeat evil and bring justice to the world.
From the first century perspective, then, the statement that 'Jesus is Lord' was about where your loyalties lay. You could follow Jesus—and "feed the hungry, care for the sick, clothe the naked, and release the captives" as Jesus demanded (Luke 4:18)—and reap an ordered, peaceful world and eternal life, or you could worship Caesar as lord and gain power and authority through empire, violence, and chaos. Proclaiming ‘Caesar is Lord’ meant you gave the glory and honor to the Emperor, that you were loyal to the power of empire and expected to reap its benefits through patronage. Proclaiming "Jesus is Lord" meant you gave honor and glory and loyalty to God (who asked you to "love your neighbor") and in doing so you would receive God's benefits.
So…who is Lord in your life?