I think the defintion of spiritual maturity is found in Hebrews 5:14 when the author writes, "Solid food is for those who are mature, who have trained themselves to recgonize the difference between right and wrong and then do what is right." This verse says that maturity is when you not only know the difference between right and wrong (knowledge), but when you actually do what is right (wisdom). This passage in Hebrews, (Hebrews 5:11-6:12) was written because these Christians were still sitting around discussing the fundamentals of the faith (i.e. baptism, the ressurection, judgement). Amazingly the people who the author was addressing were still sitting around talking about how to baptize or what hell was like. They weren't "doing" but discussing simply and fundamental principles. We equate spiritual maturity with mastering the knowledge (the theology) of right and wrong. The author of Hebrews equates maturity and spiritual formation with doing what is right. This is the difference between knowledge and wisdom. And wisdom is where you'll find maturity. 2 Peter 3:18 tells us to "grow in our knowledge of the Lord." But what is knowledge meant for? Knowledge is always synomous with a relationship to God and interaction with to others. I'm sick and tired of hearing people who always talk about right doctrine and not also right living. I'm sick of hearing people say we need to teach the world the "knowledge of the Word" and then not also discuss what the "knowledge of the Word" leads to. At some point we have to connect the two. Knowledge is always for a dynamic relationship with God and action towards others. Just my thoughts on the discussing of "mature Christians" and "knowledge".