The engine didn't really work until the priest figured it out for them.
He wasn't a priest anymore, at least if you asked the Church, but he still thought himself one. And he didn't tell them why it was failing. It wasn't an engine, either.
But Rockefeller thought it was an engine, and therefore everybody in the project called it an engine, which lead to everybody thinking of it as an engine, even the scientists. From that it followed that when they thought of engines they thought of pilots, and everybody knew the best pilots were strong, brave, of sound mind, and so on. The Consortium being the richest private enterprise on the planet, they found and discretely recruited the best of the best.
The priest himself, who was also a psychiatrist, had made sure they were.
So they took the bravest of the brave, put them in metal spheres, and used the engine-that-wasn't-an-engine to make them... go. How and where, nobody knew exactly. The pilots returned with tales of stars, empty space, and a coldness you could see. A nothing, a limbo. No matter how much the scientists tried to adjust the machinery, they could never get close enough to a star to see a planet, much less land on one.
The answer came to the priest in a dream that wasn't one. He didn't tell anybody what the dream told him. He spoke to the scientists and the financiers about "psychic waves" and the influence of "strong self image" on the engine's "guidance fields." Pure nonsense, and the scientists scoffed, but it sounded reasonable to the financiers, and they allowed him to make an experiment.
The next crew wasn't selected from the best of the best. They were desperate, unstable — prisoners plucked from Death Row, not aces selected from air forces. They returned with the description of a bleak plain and samples of earth the chagrined scientists said were not of Earth.
After that test came more. After those tests the founding of a colony. The priest went, too. Nobody commented on the fact that he was one of those the engine would transport. But he had been certain even before the first experiment. The weight of his sins was heavy enough that the engine-that-wasn't would take him to Hell. He had to go. The dream had told him he would redeem himself there by building a second Heaven to replace the one God has destroyed in His death throes, and Barrow had stained his hands with innocent blood and innocent souls, but he wasn't one to shun fate.