This is a really interesting question, and biblical application is not something that immediately comes to mind in The Phantom of the Opera by Gaston Leroux. Nevertheless, there are many biblical parallels and applications in the novel.
Let's start with the Phantom. As a young boy he was called "the devil's child"; later he calls himself the "Angel of Music." Interestingly enough, Lucifer was once an angel of music. He is one of only three archangels named in the Bible, and all were of course created by God. Ezekiel 28:13 gives us a hint about Lucifer's role in heaven:
the workmanship of your timbrels and of your pipes was prepared in you in the day that you were created.
It is clear from the totality of scriptures that Lucifer's primary purpose was as the angel of worship (music).
Like Lucifer, the Phantom has taken up residence below the world where humans live. Lucifer fell because of his excessive pride; the Phantom allowed his bitterness to keep him trapped in the underground tunnels. Both of them want what they want, and they will do whatever they can to achieve it.
Both Lucifer and the Phantom are interested in gaining control over others by offering pleasure and love. While those sound like positive attributes, both of these creatures' offerings are based on selfishness and possessiveness. They offer love only to gain control over the thing they desire.
Christine (we can't miss the reference to Christ, or at least to Christian) is the innocent one who falls for the wiles of the Phantom; though the tactics are different, Eve falls for the temptation of the devil. Both Lucifer and the Phantom believe they deserve more than they have gotten, and they are determined to right that wrong.
Raul is the most Christ-like figure in this novel. His love for Christine is pure, and he is the one who makes Christine see the truth about her Angel of Music:
“This is not the voice of your father!”
He is also willing to die for her in the confrontation in the Phantom's caverns.
Near the end of the story, Christine says this to the Phantom:
“Beautiful creature of darkness; what life have you known; you are not alone!”
In a sense, she is offering him hope, letting him know that we are all "beautiful creatures of darkness" in one way or another. This is absolutely a biblical concept, as found in Romans 3:23 and other verses:
[F]or all have sinned and fall short of the glory of God.
What is happening in the dark is a picture of the struggle to find peace (in biblical terms, to find the God of peace); what is happening in the light is a picture of the confusion caused by Lucifer (or in this case the Phantom) in order to win control of what he most desires.
After Christine leaves, she comes back to offer hope (which is the calling of all Christians) to the Phantom, bringing him the ring so he can, if he chooses to change, find real love.
The parallels are not exact, however. Consider the following:
- Lucifer fell from a perfect place and position because of his own overreaching ambitions; the Phantom did not have far to fall, which made him want what he never had.
- Christine is the one who sacrifices herself for Raul, and her sacrifice saves him--and ultimately both of them.
- The Phantom shows compassion and releases the thing he worked so hard to get; Lucifer will never let go of any soul he has won.
In general, this is a battle of darkness and light, redemption and forgiveness, false love and true love. All of these are consistent themes from the Bible.