The very first thing we do as followers of God who is Father, Son, and Holy Spirit is believe and speak. The two go together like the deity and humanity of Jesus Christ. We say, “I believe” because we are taking personal ownership of a revelation we did not invent but which God, in his grace, has revealed to us. God desires that we take that ownership and treasure it away in our hearts the way the Blessed Virgin Mary, Jesus’ first and greatest disciple, did when she “kept all these things in her heart” (Luke 2:51). That sense of personal ownership was a particularly passionate (and costly) thing in the Church of the first three centuries because it could and did get you and everyone you knew and loved killed.
In antiquity, only two groups worshiped one God to the exclusion of all other gods: Jews and Christians. “All other gods” included the Divine Caesar, who demanded a pinch of ritual incense as a worship offering and a sign that you knuckled under to his Empire. Jews generally got a pass from the Empire on this score because Romans had a policy of respecting ancient ancestral religions, including the religion of Israel and its God who commanded “You shall have no other gods before me” (Exodus 20:3).
But Christians were, in the words of Tertullian, “but of yesterday”. They had just popped up a few years ago during the reign of Tiberius. So the Romans regarded them with the same skepticism that moderns have for devotees of Scientology or some other newly minted sect and saw their refusal to worship Caesar as a combination of blasphemy and treason. The result would be three centuries of sporadic Roman persecution of Christians, climaxing in the murderous Diocletianic persecution that ravaged the Church from 303 to 313 AD.For their part, Christians took their cue from the Jewish prophets and spoke in dismissive terms of the “gods of the nations”. They said things about pagan gods that postmoderns would not at all find polite today. Pagan cultic sacrifices were offered, said the apostle Paul, to “demons and not to God” (1 Corinthians 10:20), and were rejected by Christians in language that mirrors Jewish Scripture:
Preserve me, O God, for in you I take refuge.
I say to the Lord, “You are my Lord;
I have no good apart from you.”
As for the saints in the land, they are the noble,
in whom is all my delight.
Those who choose another god multiply their sorrows;
their libations of blood I will not pour out
or take their names upon my lips. (Ps 16:1–4)
In short, to become Christian was not merely to agree to worship the one God of Israel as a sort of “lifestyle choice”. It was to renounce and reject the “gods of the nations” and to vigorously assert that there simply is no other god than the God of Israel. There was a certain quality we might today call “punk” in the blunt aggressiveness of this profession. But given the brutality to which this politically and economically powerless sect was subjected by their pagan neighbors (crucifixion, roasting on griddles, torn apart by beasts in the arena for public entertainment, etc.), it’s hard to blame them. In the early Church, Christians saw themselves as the plucky Rebel Alliance vs. the Evil Empire that was trying to keep God himself from saving the world.