Finish the proposal even if you have not finished the book. It's preferable to publishers to see a detailed, well-thought-out book proposal instead of having to read through the whole book. Once you have a commitment from a publisher, you can write your book with the knowledge it will be published when the manuscript is finished and edited.
Consider getting an agent. Agents who might handle your subject matter can be found on websites such as "Writer's Market," or "Christian Writers' Market." Attend Christian writer's conferences, where some agents look for new talent. Bring the proposal to the conference and sign up for one-on-one appointments with editors and agents.
Check market listings to find a publisher that accepts submissions from writers with no agents. While scarce, they are more common among small Christian publishers than in the publishing world at large.
Try one of the Christian manuscript submission sites. This is one of the best ways to get your proposal before the right publishers. Many Christian publishers will state in their listings that they only accept proposals through The Writer's Edge or ChristianManuscriptSubmissions. These sites have templates for proposals, and those proposals are made available to interested publishers. There is a fee for each proposal ($95-98).