Nie no Seijo to Kyuusai no Chigiri - Furyou Mahoushi to Tsuzuru Nidome no Koi

Lizelka works at a church as a saint who heals people with the highest level of magical power. One day, she suddenly loses her magical powers. Unable to figure out the cause, she begins to panic. Then, an old friend and notorious playboy, Shannon, a royal mage, shows up and forcibly drags Lizelka away, telling her to follow him if she wants her magical powers back. While researching how to get her magical powers back, she discovers that making a "promise" with Shannon might restore Lizelka's powers...!?

(Source: Comic Walker, translated)