1Corinthians 15:33 Be not deceived: evil communications corrupt good manners.
I don't know if this is the number one reason worldwide, but the number one reason I have personally seen for people leaving Christ is because they believe that after they put their faith in Him, they can return to their worldly friends and continue to participate in all the worldly events.
"I can still go to that weekend party with Bob. I don't have to drink because everyone else is."
You leave church on that blessed day feeling joyful and bubbly, and you run home to tell all your friends that you gave your live over to Jesus. They just say, "That's cool, do you want to go out tonight?" The next day you are slightly less joyful. Eventually you find yourself slipping up here and there and you don't know why. It's hard to read your Bible, and it's hard to pray. And then you find yourself just giving up.
This is why God tells us to surround ourselves with fellow Christians. (Forsake not the assembling of yourselves together) When you are around the body of Christ, you get the building up you need. You get support. You get accountibility.
Now I'm not saying don't reach out to your old friends with the gospel message; however, they are really no good for your walk with Christ if you stay around them all the time.
Surround yourselves with that which is good and true, and watch your walk with Christ get brighter and brighter.