I have to present my book(s) to a Christian publisher and I was wondering, how would I go about censoring adult language or should I just put a warning label on the title page?
You can do what authors like Robert Swindells do and substitute non-swear words for swear words. This way the dialogue can still sound realistic while being inoffensive. For example, use the word 'freaking'. Alternatively, make up an alternative word. Eoin Colfer has the fairies say 'D'Arvit'.
However, you need to consider whether it is appropriate to include actions or concepts that need adult language in a book for a Christian publisher. Also, 'Christian publisher' is not one thing. Some will only publish a very limited range of texts while others are more liberal.
I like the technique that TvTropes calls the "narrative profanity filter". The basic idea is that instead of including adult language in the dialog, you describe the language in the narration:
He speculated at length on the thief's parentage.
As a side benefit, by leaving things vague, you let the reader's imagination fill in the blanks, and you don't need to figure out what sequence of words would be involved in making a sailor blush.
In some circumstances, you could replace contemporary coarse language for a Christian audience with something like a calque of Σκύβαλον, say, 'skivalon', as this was the earthy term St. Paul used in Phil 3:8 (often rendered 'rubbish', but likely even stronger in force).
It depends on the book.
If your book is non-fiction, any bad language should be as a result of direct quotes from people. Your publisher should have guidelines about this. Ask them. 100 years ago you would have always seen dashed lines representing swearing. 50 years ago, perhaps not so much. Today, I would normally expect not at all - but your publisher should have a policy.
If your book is fiction, consider your audience. If your audience could include children, then your book should probably be edited anyway. If your audience is adults though, and swearing is relatively common in your book, I suggest that a dedicated publisher of Christian literature is probably not the place to take your manuscript.