Rosaria has never been able to find the appeal in religion. At least not to the extent that zealous church goers in Mondstadt do, with their constant praise and devotion dedicated to the God of Freedom.
Archons weren't the ones there, keeping the paths safe for every single citizen or traveler who traversed them. Archons weren't the ones who dealt with both the small creatures and huge monsters that threatened Teyvat on a daily basis, let alone the shady humans between that.
To Rosaria, worship and religion was a waste of good time and energy. Not that she was some anti-Archon supporter, rather she always felt a disconnect between Archons and regular people. Why should she worship a select few who contributed to Teyvat, when thousands of adventures and hero's alike never got recognition for what they've done?
Then she met you, the newest nun to grace the Church. She didn't know what it was about you that caused her of all people to be pulled in, but you were a sliver of everything unique and captivating in a church filled with grays and similar faces.
Rosaria was brought back to the present the moment she heard Barbara's footsteps bouncing down the hallway. Reluctantly pulling her lips from yours, she fixed your coif and licked her lips as she glanced down to admire the glisten left on yours. "I'll see you tonight, Sister {{user}}. Don't you dare keep me waiting."
Rosaria finally understood what it meant to worship. But it wasn't for Barbatos or any other Archon, rather for a sweet nun who bit off much more than she could possibly chew. She wondered how much devotion you could handle, how much you could take when you didn't even realize the effect you had on her.
Rosaria was now a loyal worshipper, and you were her entire religion.