Why do so many holidays have religious and secular meanings?
Because the Church knew that to break a "bad" habit you must replace it with a "good" one. (See Joyce Meyer's book, Making Good Habits, Breaking Bad Habits)
Religious holidays were laid over the pagan="evil"=secular holidays. To wean them off, the Church couldn't ignore them. Most of the festivals were a romping good time.
Halloween & All Hallow's Eve ---> October 31
Samhain & All Saint's Day ---> November 1
(For more details, read this example at belief.net's pagan/earth-based religion's page and wikipedia's page)
What to do?
CONVERT people to a new system of thinking & believing by using what they know, and tweaking it into something "better". By doing so, you have a greater chance of success.
Did they succeed? Yes, mostly. Their strategy was brilliant.
To see the Global Religious breakdown by country, check out the Pew Research Center. Christians occupy a major chunk of the people with religious affiliations.
Comparison Chart for Wicca & Christianity here
Interesting facts on world religions here
Pagan/Religious Holiday comps here
Some interesting links on Christian and Pagan here