After the horrifying incident in which so many people were killed by vampires and a demon leech, some members of the local church felt the need to encourage the residents of Magnolia to have hope during this difficult time and music happened to be an excellent way to draw them in. That's why they asked Gajeel and Juvia to perform after the preacher read his verses. You see aside from being a top military fighter and exorcist, Gajeel was also a gifted musician. His family had been missionaries who learned during their many travels to spread their faith, that certain pagan tribes were not very receptive to Christianity or outsiders in general. Especially when if both parties spoke languages the other could not understand.
One pagan tribe in particular attempted to kill him and his family when they first arrived but Gajeel's father, Metalicana managed to quickly appeal to them and grab their interest by playing music with his oboe. They were so fascinated by the foreign yet remarkable and beautiful sound that they spared the family and tried to learn from them.
"There are many languages that some people may never understand but the one language that we all can understand is the language of music and song." His father had told him.
So before he went into training for priesthood and exorcism, he taught himself how to play nearly every instrument there was and he proved to be a child prodigy in that subject with a preference for Celtic music. By age seven he had it mastered and he taught his musical gifts to Juvia who had been blessed with a soprano voice so heavenly it would have made angels jealous were they not pure beings.
Erza, Wendy, Lucy, and Natsu were invited to attend the church as well. The girls accepted but Natsu predictably did not. Gray contemplated going. It's not that he didn't want to he just didn't think he belonged in a holy building. He almost decided not to go in general but his Protestant faith and upbringing urged him to reconsider. But he didn't go inside, he only watched from the windows.
When preacher finished reading the passage, he called Gajeel and Juvia up to perform. Gajeel played Celtic melody and after doing an intro, Juvia proceeded to sing.
"Be thou my vision o lord of my heart
Be all else but naught to me save that thou art
Be thou my best thought in the day and the night
Both waking and sleeping thy presence my light."
Now while some of the church goers were eager to attend and listen others were more or less doing it because they had nothing better to do. A lot of these people were those who had either survived the recent attacks or lost someone they loved to the attacks and were in process of giving up. Close to saying "What's the point?" Some were barley even paying attention to the verse readings. But as soon as they heard Juvia's voice their attention was captured and focused solely on her.
"Be thou my wisdom, be thou my true word
Be thou ever with me and I with thee lord
Be thou my great father and I thy true son
Be thou in me dwelling and I with thee one."
Her voice was divine and it had a celestial harmony that none who heard it could resist. The spectators suddenly felt all their pain and doubt being washed away and replaced with a deep feeling of comfort and hope. They were all mesmerized by the nun's melody but none was more so than the devil watching her from the shadows the peaked from the church window.
"Be thou my breastplate, my sword for the fight
Be thou my whole armour, be thou my true might
Be thou my soul's shelter, be thou my strong tower
O raise thou me heav'nward, great power of my power."
As a boy Gray he had often been told that Angels from Heaven would sing the praises of God and Christ, and his innocent mind at the time often wondered what it was that angels sounded like. Now as an adult he was certain that he knew. He was certain that Juvia's voice was identical to the sound of angels singing and what light would be if it was a type of music. Also it seemed to have some sort of an effect on him. With each hymn she sung, he could feel his self loathing and guilt become hushed and turn to feelings of being welcome and accepted. For a moment he almost went into the church because he had forgotten why anyone would turn him away.
"Riches I heed not, nor man's empty praise
Be thou mine inheritance, now and always
Be thou and thou only the first in my heart
O sov'reign of heaven, my treasure thou art."
But her singing didn't just effect him. He noticed that each person in that church had gone from cynical and depressed to optimistic and hopeful. As if a switch had been turned on. But what really made him confused was that those who had survived the attacks but sustained injuries were almost instantly healed. Healed by some kind of aura that was coming from Juvia as she sang.
"High king of heaven, thou heaven's bright sun
O grant me its joys after victory is won
Great heart of my own heart, whatever befalls
Still be thou my vision, o ruler of all."
After singing that final verse nothing remained of that song except Gajeel's music and when he finished they all could not help but applaud them. All except Gray who kept silent due to astonishment and puzzlement. Astonished by just how beautiful and powerful her voice was but puzzled by why that was. There was something awfully familiar about the effects of that voice. He was certain that he had seen effects like these before although the cause of it had been quite different.
"You sounded wonderful Juvia." Erza told her after church had ended and they returned to the organization. "Growing up, I wish I had not been so busy with training to see you sing more often."
"Thank you but really the credit goes to Gajeel." Juvia said. "He's been teaching me to sing ever since I learned to talk and before that he sang lullabies to me."
"Is that true tough guy?" Natsu smirked at the priest. "What, were you like her nanny or something?"
"You find it demeaning and undignified for a man to soothe a frightened infant with music?" Gajeel he gave him a stone cold glare before pulling out a crucifix and blessed blade which both were wrapped in garlic.
Natsu recoiled and hid behind Lucy fearfully.
"No! No! No it's cool. It's manly too! Perfectly manly! Heh...Heh..."
Gajeel gave him another glare before putting his weapons away.
"Come, Gajeel." Levy said. "We have things that we must attend to at the moment. There's another quack monk running around, preaching that corporal punishment is the only way to earn God's mercy again."
"Ugh." He groaned. "It never ends with people like that. Does this one actually believe the crap he spews or is he another con artist?"
"We believe this one is genuinely delusional. Rumors say he's an escaped schizophrenic."
"Great. That means I have to go easy on him when I beat him up."
"You know our lord frowns upon finding pleasure in violent acts."
"Yeah but our lord also understands no one's perfect." He smirked.
"We'll be gone for quite sometime Juvia." Levy said. "We'll be going to Bavaria and we won't be back for at least a week."
"Will you be alright here until we get back?" Gajeel asked.
"You know I'll be fine. I've been staying here by myself." Juvia said.
"Yes but we were still in the same country with you and you'd be able to reach us with a phone call."
"You don't need to worry. I'm a lot more capable then you give me credit for and remember you're the one who trained me in exorcism so you know that I'm well taught."
"That's true. I am the best there is." He chuckled.
Levy winked at Juvia as way to show how impressed she was at how she had learned to get Gajeel to comply with things by stroking his ego.
"Alright I guess you can handle yourself. But keep your gun and your cross with you always." He told her. "And promise me that you won't be alone with anything that's supernatural or a man."
"I promise you that when you return I will still be alive, human, and a virgin."
"So you promise that you won't be alone with anything not natural or female?"
"Goodbye Gajeel." She said ushering him along.
"Just remember that men are animals! Especially eternally youthful ones because their hormones never go away!"
"Goodbye."
She bid farewell to them and they left for their trip.