You could you both spellings as sudhani suggested, "Christian Website : Web Site for True Christians." On interior pages you could have one page for each term. Regardless you may want to explore other options.

I would concentrate on other keywords. When searching on an engine would you necessarily use "website" as a keyword anyway? Except for website design, etc. most people wouldn't use that as a search term. Isn't it a little repetitive, of course you're a website, that sums up ALL internet webpages. How about concentrating on a list of words that aren't as competitive and more realistic on what people would be searching for.