"They're only truly great who are truly good." - George Chapman

This quote means that if you are a good person, then that makes you a great person because if you do something good, it will make you a great person and that you will keep on being a great person if you do good things to others or for others or even for yourself. For example, if you do something bad to a person, then that makes you a bad  person and bad things will return to you when you did something bad to someone else, and if you do good things to or for others, then that good of you will lead you to good things and that might even return something good to yourself, just for doing something good for others or to others, and that's choosing the right way or the right thing in life.