Paul says that the gospel brings to light the mystery that had been hidden in God, and that this revelation of the mystery aims to make the wisdom of God known to “the rulers and the authorities in the heavenlies” (Ephesians 3:8-10). Who are these rulers to whom the mystery of the gospel is directed?
Isaiah helps answer that question. When he begins outlining the work of the Suffering Servant, he says that the exaltation of the Servant will astonish rulers: “Kings will shut their mouths on account of Him; for what they had not been told they will see and what they had not heard they will understand” (52:15). Rulers are the target of the prophet’s message about the Servant. The gospel of the Suffering Servant is proclaimed to kings .
It’s not entirely clear how the story of the Servant has its effect on rulers. Pilate was amazed at Jesus’ silence in his court, and we may surmise that rulers would be astonished to learn that the path to glory is not the path of head-crushing war but the path of suffering faithfulness.
So when rulers (say, Constantine) start responding to the gospel, it is not a deviation from the program. On the contrary, sending a message to kings was a central point of the gospel. And when they respond, we’re seeing the fulfillment of ancient prophecy.