Nothing really wrong with what you said but I'd shorten it a bit.
Imagine you have a friend who all knowing, almighty, and who loves you infinity. That friend is God.
The second line for me has a logical problem you don't develop to a reward. Secondly it can only be rewarding if you do it, so its better to say can be rather than is.
So I'd probably rephrase it like this
Developing a personal relationship with God can be the most rewarding process in our lives.