The character was in training to be a cleric. He read about this fallen angel in the holy scriptures and got curious about it. As he dug deeper, he became convinced that the scriptures had been tampered with by enemies of the faith; the angel wasn't fallen at all. When his efforts to establish contact with the angel and prove his point were discovered, he was thrown out and excommunicated for heresy. (To his superiors, it looked like he was trying to make friendly contact with powers of the lower planes, because, well... he was.)
He didn't give up, however. He continued striving to make contact with the fallen angel, trying darker and darker magic to do so, and finally succeeded. The angel told him yes, it's all true, I never fell, and I need you to clear my name and restore my worship. But be careful, the church is under the control of our enemies; you have to build support secretly, and there may be bloodshed before it's all over. It couldn't give him true clerical magic, but it could and did bestow warlock powers on him instead.
So, he is now trying to infiltrate his old church, establish a demonic cult, and kill the church elders, all while firmly convinced of his own righteousness.
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