Steve is almost an ideal father. Even though he is dying, he keeps his illness a secret and puts the wants and needs of his children ahead of his own. Through his words and actions, Steve demonstrates to his children, but most significantly to Ronnie, what it means to love another. And Ronnie is able to learn from her father's words and actions as she grows into an upstanding young lady.
Many questions remain about Steve's actions surrounding the divorce — Should he have left his family? Should he have confronted Kim about her affair? Should he have let Ronnie know the truth about his reasons for leaving? But just as Ronnie takes a journey of maturation, Steve is on his own journey: his quest is to find God.
In a sense, Steve is an "Everyman" who is facing his mortality and trying to attain spiritual and emotional peace before he dies. His questions, uncertainties, and fears are those of all people, but particularly of those who realize that they will not be alive much longer.
In another sense, Steve is a symbol for God's love. Steve, like God, loves unconditionally, trusts in his children, and knows his children better than they know themselves. His words and actions model the essence of love. Just as Steve yearns for the presence of God in the world, not realizing that he was experiencing it during the summer with his children, Steve becomes a symbol of how to live your life, though Ronnie is also too blind to see it until the end.