Determine your book's value as a mainstream publication. Is it overtly Christian and about Christianity, or does it merely have strong underlying Christian themes? If your book is of a genre and contains material such that it might be popular or useful to non-Christians as well, you can submit it for publication to both Christian book publishers and mainstream publishers.
Look for published books similar to yours. Visit the shelves of libraries, mainstream bookstores and Christian bookstores and locate books that are similar to yours in genre, style and content, then look to see who publishes them. This will help you determine which publishers are a good match for your book.
Get a literary agent. While authors can submit to publishers directly, having a literary agent always helps this process, especially since some publishers will only take queries from agents. See Resources below for some agencies that represent Christian writers, and follow their instructions for getting representation. It usually helps to have an established reputation as an author, such as having some shorters works published in magazines or anthologies, or maintaining a popular Christian web log.
Submit queries to publishers you've identified as appropriate to your Christian book (see Resources below). Each publisher will have their own guidelines for submission; always follow them to the letter.
Consider self-publishing if you already have access to the means to share your book with an audience (such as a local bookstore that you know will stock your book or an existing audience of readers on-line). The electronic age has made this option easier, more affordable, and more likely to be successful than other for aspiring authors. Check out a print-to-order company, or publish your writing in ebook format for download. With self-publishing, there's no need to use a specifically Christian company, as you will handle the promotion of your book yourself (see Resources below).