"The Kingdom of God is at hand: Repent!" In the first century, Jesus and Paul proclaimed this same message, Jesus to fellow Jews, Paul to pagans. Jesus defined the sins of his audience by appeal to the Ten Commandments. The primary sin of Paul's audience, by comparison, was idolatry. Their mission fields were different, but their urgency was the same. God, they taught, was about to intervene in history, raise the dead, defeat evil once for all, and establish his kingdom of peace. The Kingdom was at hand.

But the kingdom did not arrive in the first century; and Christianity as a post-apocalyptic movement continued to develop in vigorous variety. As circumstances shifted, so did the questions that concerned different churches. Did Christ even have a fleshly body at all? No, insisted some second-century Christians. Was the god depicted in Genesis actually the father of Christ? Most second-century Christians thought not. Did God really say, "Be fruitful and multiply"? Virginity and celibacy, replied later Christians, are actually what he prefers. Is the resurrected body a body of flesh, or a body of spirit? Spirit, answered some, while others answered, Flesh. And where would these raised bodies go? To Jerusalem, urged one teacher. To the upper cosmos, taught another. To a purely spiritual realm, replied a third. Who are the redeemed? Everyone, taught Paul, mid-first century: the full number of the gentiles and all Israel. Almost no one, taught Augustine, early fifth century: many more are condemned to damned darkness than are predestined to eternal light.

"Apostle Paul" by Rembrandt at the National Gallery of Art, Washington DC

I am an historian of ancient Christianity. For the past thirty years, in the course of a fourteen-week semester, I have acquainted my students with a sense of its growth, its development, and its dynamic complexities. But in 2006, Princeton University's Committee on Public Lectures challenged me to do the same, for a (mostly) non-academic audience, in the space of three hours spread across three evenings. Impossible, I thought. Yes, I said. And then, of course, I panicked. HOW?

"Sin," it turned out, was my salvation. Ideas about sin are like radioactive isotopes: you can trace them as they course through different theological systems, revealing their social bony structures, their ideological soft tissue and muscle mass, their intellectual blockages. By attending to ideas of sin, I could trace as well their correspondence with and to other crucially important ideas - ideas about humanity, and what it is to be truly human; ideas about the universe, and humanity's place within it (or beyond it); ideas about Christ as redeemer, and ideas about who Christ redeemed, and how.

The biggest surprise was God. Ancient Christians' ideas about God displayed the same intense mutability as did their ideas about sin. "God's" changefulness truly caught my audience up short: after all, didn't the Bible present a common quarry for all Christian constructions of God?

The answer, of course, is Yes and No. Jesus and Paul did not read the same Bibles: Jesus' biblical tradition was in Hebrew or Aramaic, Paul's in Greek. All translation is already a species of interpretation: the Greek and the Hebrew of the Jewish scriptures do not say the same thing. (The gods of the nations, in Paul's texts of Psalm 95.5, were daimones, "demons": demons for his pagan audience were lower gods, but gods nonetheless.) Their different circumstances and environments - Jews or gentiles, within the Jewish homeland or out in the cities of the Graeco-Roman diaspora - determined which aspects of the Bible mattered most to these different audiences. The God of Israel had more divine opponents when the Bible was read in Greek.

But even once everyone read the same Greek text of the Jewish scriptures - as was the case with the three second-century figures that I presented - the variations only multiplied. These later theologians thought with the categories of philosophy. For them, the highest god was by definition changeless, radically transcendent, absolutely beyond any direct interaction with humans, who moved through time and space. The god of Genesis, by contrast - in fact, the god presented throughout Jewish scriptures - was constantly doing things: framing the universe; creating all life in it; appearing to Abraham, chatting with Moses, advising Isaiah. Who was that deity, then? The god of the Jews was one logical answer; but such a lowly, history-constrained god was obviously not the father of Christ. His divine father had to be a god above the biblical god. No! responded a rival Christian. The high god of philosophy was indeed Christ's father. Therefore the god who showed up in the Jewish scriptures had to be Christ, before his incarnation. In this way, the heroes of the Old Testament "became" worshipers of the pre-incarnate Son: Abraham, Isaac, Jacob, David, claimed this theologian, had actually been Christians.

But the biggest contrast in ideas about God appeared in the work of two later theologians, Origen of Alexandria and Augustine of Hippo. Both men read the same Bible, Old Testament and New Testament; both had the same allegiances to orthodoxy. Yet according to Origen, all humanity - indeed, all of God's creatures - would be saved; according to Augustine, the vast majority was damned. According to Origen, since God is just, he gave humanity free will so that a person could choose whether or not to sin. According to Augustine, since God is just, he condemned all humanity to a broken will as part of the price of Original Sin. According to Origen, God loved all his creatures, even Satan: at the End, accordingly, even Satan would repent, and at last be redeemed. According to Augustine, God is angry at humanity on account of the sin of Adam. He graciously redeems just enough (though they do not deserve redemption) to show forth his mercy; but he justly condemns most - even babies, if unbaptized - to display his justice.

Augustine died alone, reading the penitential psalms, weeping for his sins, going to the inscrutable and angry god that he had created. Ideas about sin, meanwhile - and thus, ideas about humanity, about the world, and about God - have not stood still in the sixteen centuries between his age and ours. And raucous variety characterizes modern constructs of sin no less than it did the ancient ones that I reviewed (though modern people seem most often to identify "sin" in others, couching their own malfeasance in the language of "mistakes"). Context, with these religious ideas as with all others, intimately affects content. At the end of the day, howsoever defined, "sin" suits its times.