The Vows We Make by Sonya Loveday

Mark and Paige are living in sin—that’s what any man of the cloth would say. All they want is to get married, but they refuse to exchange vows without their best friends—the Six. As they wait to hear back from them, Mark and Paige can’t help but feel they’ve been put off… forgotten.
When the threat of an enemy stalking them is brought to light, the call is made to bring them into the secretive world of Cole

Enterprise.
Slotted for recruitment by the very agency that stole their chance at a normal life, Mark and Paige have to adjust to their new path as secret agents in a war they didn’t even know existed.
What neither fully understands is that nothing will ever be the same… and some vows are deadlier than others.