Joshua Harris says in Dug Down Deep: Building Your Life on Truths That Last," Theology matters, because if we get it wrong, then our whole life will be wrong...We're either building our lives on the reality of what God is truly like and what he's about, or we're basing our lives on our own imagination and misconceptions. We're all theologians. The question is whether what we know about God is true."
Although Harris' book is not earth shaking in the slightest, its intent is to "tak[e] doctrinal ideas off the high shelf...and put them to work in [our] everyday lives" (25). He attempts to show through Scripture how knowing God's personal attributes is imperative, why Jesus had to be made flesh, why the Cross is necessary for salvation, what is the difference between salvation and self reformation, how holiness isn't just a list of rules to keep unbroken, why fellowship with a church body is important.
Where some authors forget to say why he believes something, Harris litters the text with Scripture references, sending you scampering back to God's Word to check it yourself. It's worth tracking down his book if after reading this, that little nagging voice in the back of your head says you don't have a firm a grasp on the foundational truths as you come across to others as having.