Also: Faithfulness, Loyalty, Fidelity
Devotion is loyalty to a promise, vow, person or mission. It means you are consistently trustworthy and loyal. You are willing to sacrifice to uphold your commitment. It means staying the course, even if the challenges are great, and even if you find your energy and will weakening.
When you are are devoted to someone or something, the object of your devotion become a part of you. Your own sense of well-being depends on whether they are flourishing. This makes devotion risky -- but also profoundly rewarding.