Off Armageddon Reef

Humanity pushed its way to the stars—and encountered the Gbaba, a ruthless alien race that nearly wiped us out. Earth and her colonies are now smoldering ruins, and the few survivors have fled to distant, Earth-like Safehold, to try to rebuild. But the Gbaba can detect the emissions of an industrial civilization, so the human rulers of Safehold have taken extraordinary measures: with mind control and hidden high technology, they've built a religion in which every Safeholdian believes, a religion designed to keep Safehold society medieval forever.
800 years pass. In a hidden chamber on Safehold, an android from the far human past awakens. This android, Merlin, emerges into Safeholdian society to begin the process of provoking technological progress, which the Church of God Awaiting has worked for centuries to prevent. To conceal his formidable powers he finds a base of operations in Charis, a mid-sized kingdom with a talent for naval warfare, where he plans to make the acquaintance of King Haarahld and Crown Prince Cayleb, and maybe, just maybe, kick off a new era of invention. Which is bound to draw the attention of the Church...and, inevitably, lead to war.
It's going to be a long, long process. And David Weber's Off Armageddon Reef is going to be the can't-miss Sci-Fi epic of the decade.