By Heresies Distressed

In David Weber's By Heresies Distressed, the battle for the soul of the planet Safehold has begun.
The Kingdom of Charis and the Kingdom of Chisholm have joined together, pledged to stand against the tyranny of a corrupt Church. The youthful Queen Sharleyan of Chisholm has wed King Cayleb of Charis, forging a single dynasty, a single empire, dedicated to the defense of human freedom. Crowned Empress of that empire, Sharleyan has found in Cayleb's arms the love she never dared hope for in a "marriage of state." In Cayleb's cause, his defiance of the ruthless Group of Four who govern mother Church, she has found the task to which she can commit her mind and her courage. It is a cause for which she was born.
Yet there are things Sharleyan still does not know. Secrets Cayleb has not been permitted to share, even with her. Secrets like the true story of humanity on Safehold. Like the intricate web of lies, deception, and fabricated "religion" which have chained humanity for almost a thousand years. Like the existence of the genocidal alien Gbaba, waiting to complete mankind's destruction should humans ever attract their attention once more. Like the existence of a young woman, Nimue Alban, nine hundred years dead, whose heart, mind, and memories live on within the android body of the warrior-monk she knows as Merlin.
And so Empress Sharleyan faces the great challenge of her life unaware of all that task truly entails...or of how the secrets the man who loves her cannot share may threaten all they have achieved between them...and her own life.