G. Reale, A History of Ancient Philosophy: The Schools of the Imperial Age, trans. John R. Catan (Albany: State University of New York Press, 1990) 416-417.
Let us see in the first place what is the exact definition Iamblichus had of theurgy. In the De mysteriis it is presented as a practice and an art with which, by means of appropriate acts, symbols, and formulas not understood by human reason but understood by the Gods, man can join himself to the Gods himself and benefit from their influence and power...
Hadot clarifies these concepts very well: "If we could achieve the perfect union with the Gods by means of contemplation, then it would be by our powers that we could reach the divine. The God would be then moved by lower beings. On the contrary, if they themselves choose the practice, incomprehensible to men, by means of which it can be hoped to be united to them, they remain immobile in themselves and maintain their initiative."
Reale often states that the concept of grace was radical in the philosophical schools of this period (second-fifth centuries C.E.). After all, Plotinus, believed that human beings could reach God through things that they did.
At some point, however, there emerged the belief that the gods were impassible, or unresponsive to humans. But advocates of this view did not go the route of the Epicureans, who thought that the gods didn't care about us and so we shouldn't care about them. Rather, they still craved union with the divine. And so they concluded that the gods are the ones who take the initiative to bring us closer to them, meaning that we don't climb up to them through our own efforts. The belief in the gods' impassibility led Neo-Platonists to embrace a notion of the gods' grace.
At the same time, they acknowledged means of grace: mysterious rituals that the gods have devised to bring us closer to them. That's called theurgy, and it doesn't always accord with our human reason.
Christianity has a lot of these same issues. Calvinists like to emphasize God's sole part in bringing us to believe, since that is their definition of God's grace. In their eyes, if a relationship with God is based in any way on our efforts (e.g., believing, good works, etc.), then we are climbing our way up to God, which leads to self-righteousness rather than a humble reliance on God's grace. But believers in predestination do believe that there are means God has commanded that are conduits for his grace: baptism, the Lord's supper, the preaching of the word, etc.
Moreover, Iamblichus' view that theurgy can conflict with our reason reminds me of what Paul says in I Corinthians 1:18ff.: God used an outwardly foolish means to save human beings. In Paul's day, the cross of Christ made sense to neither Jews nor Gentiles. But that was how God chose to save humanity. And Paul stresses, like Hadot, that human wisdom did not bring people to God.
One would think that God would meet us at our level, by giving us revelation that makes sense and accords with our reason. In many cases, he does. But maybe he's more interested in the type of people that we become than in whether something makes perfect sense to us. Perhaps God wants us to be humble and dependant on his grace, not proud of our ability to climb up to him.