Dangerous Sanctuary by Shirlee McCoy

Mission: rescue his fellow agent.

FBI agent Radley Tumberg must rescue his fellow agent, Honor Remington, from a spiritual sanctuary where she's being held against her will. But when he reaches her, posing as her estranged husband, he discovers the motives for her capture are deadlier than he expected. Can they escape the sanctuary and find evidence that its leader isn't what he's pretending to be?