Defender of the Flame by Sylvia Engdahl
Starship pilot Terry Radnor is elated to be among those chosen to defend the secret colony Maclairn against enemies who pose a threat to the spread of paranormal human mind powers. He commits himself wholly to the goal of that world, not guessing how far his effort to safeguard it will take him from everything else he cares about–his promising career as a Fleet officer, contact with people who share his newly-discovered psi capability, his wife and unborn child. Torn away against his will after learning a secret too deep for its disclosure to be risked, he is forced into exile from all that matters to him, and must build a new life grounded far from Maclairn. There seems little hope that he can fulfill his pledge to protect the world whose safety is crucial–more crucial than anyone else knows–to the future of humankind. Yetan extraordinary destiny has been predicted for Terry, and fate puts him in place to confront the colony’s greatest peril.