1. How can Christian themes be incorporated into (insert plot premise here)? Even if it IS a good story, God's glory needs to be the focal point.
If you're a practicing Christian, they're going to show up in your story no matter what. One of the stories I just finished working on for the time being started out with me not purposefully saying "This needs to be an overtly Christian story," but Christianity ended up becoming a central part of it naturally anyway. Just don't think about forcing it in there because that won't work (been there, done that). "Christian themes" can mean a lot of things, a lot of very broad or very specific things. Sacrificial love, helping the poor and oppressed, humility, grace, and forgiveness are examples of broad Christian themes. They can and do exist outside of a Christian context, but they line up with what Christianity teaches nonetheless.
If you're thinking in more specific terms, attribute all of the above to God in the story. Characters can forgive because God forgave first, stuff like that. Something that always happens to me no matter what is that my characters, at the very least, acknowledge the existence of God and/or a reality/creator greater than themselves.
You could dedicate your writing to God. I did that a few years ago and ever since, every story I flesh out has involved him somehow and Christianity naturally weaves itself into the fabric of the story.
Something to avoid is the stereotypical Christian goody two shoes protagonist who converts everyone to Christianity and is always happy and loves God all the time. This isn't realistic and it's just a recipe for Mary-Sues and all their relatives.
2. Can this be made into a novel-length story? I'm looking to write a good-sized story. If I wanted to write a short one, I''d have just gone and done it.
If you want it to. Any plot bunny can grow into a novel. If you're having trouble, consider this. All stories can be broken down into cause and effect. Something happens that changes the status quo and there are consequences (good and/or bad) that cause more things and lead to more effects.
There are about a thousand different ways to plot a story, but starting with an outline is generally a good idea. Sometimes, starting the actual writing is faster/better. It depends. Even if you have an outline, you're story's going to deter from it but at least you have a general idea of where you're going.