To be spiritually mature is to both grow and show the fruit of the Spirit in our lives: the love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control listed in Galatians 5:22,23.
In these studies we see the fruit illustrated in the lives of characters from the Bible - the joy of the disciples at seeing their risen Lord; the faithfulness of Jehoshaphat when facing a huge enemy army; the self-control of David in resisting revenge on Saul.