St. Augustine asserts:
[a] that the Roman Empire’s rise was an act of Providence by the true God to prepare the way for Christianity;
[b] hence the Empire’s rise cannot be attributed to the local gods—who are “devils” or “demons”, false gods;
[c] for Augustine, the true God is utterly separate false gods, because he is good whereas they are evil;
[d] yet this means that when a Roman general prayed to Mars, gained courage thereby, and then won a battle that the true good God worked through the false evil god;
[e] hence if, for example, I pray to a pagan god there is no way a Christian cannot say that in some way that is not part of God’s plan, the workings of Providence;
[f] yet Augustine says that these false gods must be suppressed, even though he admits they ultimately serve the true God.
The contradiction is related to another problem in Christianity, more commonly stated, which is that the all-powerful good God gives you free will to do evil acts, acts which Augustine says have no connection to God; and yet if that is so then the good God is not all-powerful, since there are acts outside his province. The error is connected to the Zarathustrian dualism that informed Christianity—in Zarathustrianism the good god and evil god are totally separate.
Essentially, the purely good Christian God is all-powerful; and yet, if he is all-powerful, he must in some way be implicated in evil acts—and yet Christians insist he is not implicated in them at all, hence he cannot be all-powerful.
The first contradiction is related to the way Augustine tends to say, post facto, that whatever happened was the will of God—and, therefore, for the best. Hence, if the battle is won, it was the will of God to defeat the wicked pagans and so destroy them—but, if it is lost, then it was God’s will to chastise the Christian backsliders and so improve them.
This is a tautology, but it allows Augustine to say, in fancy language, “it happened because it happened”—a statement that explains nothing but, given suitable verbal decoration, may comfort, say, the parents of a child that just died.
Unfortunately, it also leads him into some ridiculous statements; for example, he believes in collective punishment—he says that the reason virtuous Christians and wicked pagans both suffered when Rome fell was that the virtuous Christians were to receive a salutary caution not to be attached to “the life of this world”, and also to serve as an example in suffering to the wicked pagans.
This is manifest injustice—but it reflects Augustine’s tautology; it happened because it happened and because it happened it’s just—you can see how the Jews, as a slave race, thought up this ethic as they put up with being shifted from one master to the next; but it’s unjust and leads to a passive attitude to life and a general lassitude—it also explains nothing.
The main point is that Augustine basically says that the one true and good God works through “untrue demon gods” to attain his ends—hence it is impossible for a Christian to say, when I worship my statue, that I am not secretly doing God’s will; and so they should leave me alone.