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.

Imagine you have a friend who knows everything ; who is almighty and yet loves you with infinite love. The friend is God. Developing a personal relationship with God is the most precious reward that we can receive in our life> Developing an intimate relationship with God is the most precious thing that one can do in one's lives. Developing an intimate relationship with God is the most rewarding thing one can do with one's life