Thomas Aquinas is without question one of the greatest theologians in Christian history. Catholic and Prostant theology alike have formed either with or against him point by point. Therefore, in attempting to talk meaningfully, and theologically, about the Christian life, Thomas is a good guy to dive in to (if you have the stomach for it!). If you are so inclined, I have a recommendation. Nicholas Healy has written a book entitled: Thomas Aquinas: Theologian of the Christian Life. This is one of the volumes in the Great Theologians Series, which seeks to offer introductory overviews of a thinkers thought (and has become my starting place to get into a thinker - the line is fantastic). If you have never sat down with the Summa and want to, let me suggest you start here. And even if you never want to read the Summa but you find youself wanting to think about more theological issues, Healy provides Thomas' thought in a readable (and short!) account.