The church made an ungodly mess of things when it had power. The crusades, inquisition etc. are just the tip of the iceberg. In general, it wanted to control people and resisted any real change. Most of all, its sheer complicity in power and wealth is astonishing. And in all of this the teachings of Christ get lost. Not that there was no-one trying to follow Christ, not that the project of 'church' is to be written off. But Harris' novel does, at the very least, suggest that we are reaping the evangelistic effects. And they're not good...
RS Thomas puts it this way:
"... On saints'
days the cross and
shackles were the jewellery
of the rich. As God
aged, kings laundered their feet
in the tears of the poor."
(from 'Bleak Liturgies' in A Mass for Hard Times.)
The villain of the piece is not Constantine. The church was, in fact, rather good at maintaining some independence from the Roman Empire (viz. Ambrose, John Chrysostom, Desert Fathers and Mothers). In the West, at least, the church did rather well out of the Roman state, and was not required to compromise everything. Indeed, the church outlasted Rome, and was the source of stability and learning as the world fell apart. And it is here, not in the conversion of an emperor but in the dangers that accompany success, that the problems. Here in the church as an undeniably good thing, here is the temptation and the failure, in the enjoyment of the power and wealth that accrued to the church as she did her job rather well.
God preserve us from success.