Spiritual Growth is based on one's enlightenment and development of understanding within a particular practice based on ones spirituality. There are many forms of spirituality and beliefs. When we develop spiritual growth we find our selves integrating and applying our spirituality on a higher level in our daily lives.
A good analogy to illustrate the idea of spiritual growth is a lit lamp. If we view our body as a lamp the more awareness and consciousness we experience the brighter our light becomes as a lamp. The brightness we emit sheds light so we can see our selves and our surroundings more clearly. We may see things in the light we never saw before or understand them differently. This is spiritual growth. The brightness we give off also may enable us to see into other levels of existence and consciousness. This is also part of spiritual growth.
Some ideas of spiritual growth overlap with religious practice. Other ideas of spiritual growth may be more metaphysical in origin. The primary idea that weaves through all the different forms of spirituality and spiritual growth is the belief of losing ones personal focus of self and I, and focusing on a larger sense of creation and being in harmony with things larger than ourselves. Some people call spiritual growth tuning into "God" or a Supreme Being or Creator and going by the guidelines that it manifests. When one is in harmony spiritually with that idea that there is something outside of ourselves that can make us enlightened when we seek to understand our relationship to it, we experience spiritual growth.
Spiritual growth does not have to be founded in the association of a particular religious view of what god is. It can be more universal than the confines of a particular religion. You may be of a particular religion that encompasses spiritual growth as well, but you don't have to be necessarily to experience spiritual growth. You can still grow spiritually without practicing a particular religion. All religions claim if you follow their teachings you will grow spiritually and receive the benefits of that faith. This is fine but there are people who don't believe in an organized faith or believe in more than one organized faith and still experience spiritual growth.
The main thing to consider for spiritual growth is good character regardless of ones belief. Are you kind, giving, sensitive, understanding and helpful to others in the world. Or, are you selfish, only interested in your self-gains, and narrow minded not allowing for others to live in harmony in the world as well. If you are the second type of person then it may be safe to say you probably won't experience too much spiritual growth.
Spiritual growth is synonymous with other terms like spiritual development, development of higher awareness or development of higher consciousness.