In the category of preaching to the converted:
Each book of the Harry Potter series is imbued with great Christian lessons. We might argue over Rowling as stylist or Rowling as successor to Tolkien and Lewis or Rowling as literature; however, to the reader who has spent any time with the books, Rowling as devout and informed Christian is nowhere in doubt. Each book teaches something about the believer in Christ and how that believer behaves in certain circumstances.
The particular event of interest occurs at the end of the fourth book of the series, Harry Potter and the Goblet of Fire. It is spectacularly portrayed in the movie, and caps the book off with a scene horrifying, dramatic, and stirring. Harry Potter and Cedric Diggory have both touched a device that transports them to a place where the bane of the series Lord Voldemort await the arrival of Potter. Upon arrival, Cedric is summarily dispatched and Harry's blood is used to revivify the skeletal, embryonic Voldemort.
Then ensues the duel in which Voldemort attempts to finish off what he began so many years ago--the death of Harry Potter. The two engage.
Now the remarkable instance--in the course of the engagement Harry sees Cedric, Harry's mother and father, and (in the book, if I remember correctly) a whole host of those whom Voldemort has killed over time. Harry's mother tells him, "We can only give you a little time." The host descends upon Voldemort giving time for Harry to run to Cedric's body and transport the two of them back to Harry's world.
If, in this instance, we allow Voldemort to stand-in for sin, which, as we know from St. Paul leads to death (hence the derivation Vol-de-mort or "flight of death"--which will have several meanings in the series) we can see the communion of the Saints as it works. We engage in a battle with sin, temptation. We are the combatants. The fierceness of the battle and our faith summons help from Heaven's throneroom, the Saints, who engage through prayer the powers, principalities, thrones and dominations, that trouble Heaven and our own world. As Harry's mother advises, they can only give respite, it is up to us to flee from sin--but they can and do intercede for us providing the out--we can escape if we move away (of course aided by the Saints and God's will).
This image is reinforced later when Dumbledore, unpacking the experience for Harry, reminds him, "You know, we can never bring back the dead." Harry doesn't seem to understand this for what it means, but it is very clear to the reader that we cannot bring back the dead because, in fact, they never leave us. They are a cloud of witnesses gathered about us thickly and participating in every event of our lives--those tied to us by blood, most fiercely, but aided by all the warriors of Heaven (It is my hope that, undeserving as I am, the chiefest of those warriors is the Holy Mother of God and the Great Redwood of God, St. Therese.)
Thus, embedded, entangled, and completely blended throughout her series of novels, Rowling gives us lessons and views of how Christianity really operates. "But no one ever goes to Church or prays, or anything Christian." And of course, as anyone knows, that is less than nothing as an objection because the same holds true for both Tolkien and Lewis, her forbears in the art of bringing the truth of Christianity to the unsuspecting reader.