Think about it, everything we do has some kind of reward (or perceived reward) attached to it.
Even "selfless acts" make the person commiting them feel good, and that's why they do them. we are slaves to our own pleasure, however that pleasure manifests itself.

Christianity preaches that God gave man free will, but is this free will truly free if we can do nothing but serve our own intrests?

If you didn't have free will, would you know it?

Now, you could try to prove me wrong, by doing something rewardless but what purpose would that serve? Well you'd have proved me wrong. There's your reward. not a great reward, but it is one none the less.