The "Edict of Milan," whose milleseptuacentennial (so to speak) is being marked this year, wasn't an edict and wasn't issued at Milan. Still, its enormous impact on the history of the Church and the West is well worth pondering on this 1,700th anniversary.

In his magisterial study, ''The First Thousand Years,'' Robert Louis Wilken sets the historical record straight:

"In ... 313 ... Licinius (the eastern Roman emperor) and Constantine (the western emperor) had met in Milan. The occasion was the marriage of Licinius to Constantine's half-sister, Constantia. But the two emperors used the occasion to discuss matters of state and agreed on a policy concerning the practice of religion. During the summer of 313 Licinius sent letters to provincial governors ... in the East, in Asia Minor and Syria, granting Christians the rights they had already acquired in the West and restoring their property. This letter has often been called the "Edict of Milan" but the term is a misnomer. It was not an edict but a letter posted by Licinius from several cities in the East, such as Nicomedia, the residence of the emperor. Like other official correspondence, however, it was written in the name of both emperors and its content reflects the hand of Constantine."

Licinius's letter, Wilken notes, involved all religions, not just Christianity; it went beyond mere toleration and embodied a more robust idea of religious freedom, based on the conviction that true faith and true worship cannot be compelled; and it treated the Church as a corporate body with legal rights, including property-owning rights. Thus the not-really-an-Edict of Nicomedia and Elsewhere cemented into the foundations of the West ideas first sketched by the Christian philosopher Lactantius: that coercion and true religious faith don't mix because "God wishes to be adored by people who are free" (as Joseph Ratzinger would rewrite Lactantius a millennium and a half later, in the 1986 Instruction on Christian Freedom and Liberation). The rather humane provisions of the mis-named "Edict of Milan" were not infrequently ignored in subsequent western history; but that doesn't alter the fact that the "Edict" had a profound and, in many respects, beneficial influence on the future of the West.

There was a shadow side to all this, however. For what we know as the "Edict of Milan" marked the beginning the Christian Church's deep entanglement with state power.

The immediate effects of the Constantinian settlement, both good and ill, were limned with customary wit and literary skill by Evelyn Waugh in the novel ''Helena.'' After 313, the tombs of the martyrs were publicly honored; so were the martyr-confessors, often disfigured by torture, who emerged from the Christian underground to kiss each others' wounds at the first ecumenical Council. Before those heroes met at Nicaea in 325, though, grave theological questions had gotten ensnared up in imperial court intrigues and ecclesiastical politics. Later unions of altar and throne led to a general cultural forgetting of Lactantius's wisdom, as the Church employed the "civil arm" to enforce orthodoxy. Protestantism proved no less vulnerable to the temptation to coercion than Catholicism and Orthodoxy; one might even argue that the 17th-century Peace of Westphalia, which ended the European wars of religion by establishing the principle of cuius regio eius religio (the prince's religion is the people's religion), reversed the accomplishments of the "Edict of Milan"--and was, in fact, the West's first modern experiment in the totalitarian coercion of consciences.

Very few 21st-century Christians would welcome a return to state establishments of religion as the accepted norm. So however much the Constantinian settlement led Christianity into what some regard as a lengthy Babylonian captivity to state power, the "Edict of Milan" also affirmed truths that have proven stronger over time than the temptation to use Caesar for God's work. Today's challenge is quite different: it's the temptation to let Caesar, in his various forms, reduce religious conviction to a privacy right of lifestyle choice.

Lactantius knew that religious conviction is more than that. Seventeen-hundred years later, so should the Obama administration and the West's radical secularists.

George Weigel is Distinguished Senior Fellow of the Ethics and Public Policy Center in Washington, D.C.