I belong to a local church body where the vision is:
"We are pursuing Christ's reign in all of life to advance God's kingdom in the all the earth for His glory."
It's a big vision. It's is a good vision, one worthy of devoting the rest of my life to. By God's grace I pursue the reign of Christ in all the areas of my life - for His glory and my joy.
The leadership at my local church has been working hard to spread this vision within our body. With a vision such as this, the implications are many and working them out to fullness will no doubt change lives.
I think one of the implications of this vision is to evaluate the maturity level of one's own understanding of who God is and what exactly is His plan for the world. Another way of coming at this is to think about the importance of theology in the layman's life. I don't think the necessity of well-formed theology is just for "those in the ministry." All who pursue Christ's reign in all of areas of life are in ministry (full-time) and so then theology is incredibly important.
Theological knowledge comes from many different sources. On the macro level I think there is a continuim that illustrates where people operate and how they go about growing in theological knowledge. On one side there are formal methods that require heavy committment and involvement such as classes, seminars, workshops, and long-term bible studies. On the other side there are lower committment level methods such as devotionals, books, and sermons.
I do think a sermon does contribute to a person's theology but I think it is limited in what it can actually accomplish. The issue comes down to taking ownership in understanding what the Bible actually says and developing personal theology.
Sermons should not be the sole source of one's theology. A sermon can change a person's life but the purpose of a sermon itself seems to be to encourage and exhort. Sermon's are more one way delivery and there are not immediate opportunities to argue, discuss, or debate what is being said. Because of this, people listen more and are probably more likely to hear God speaking to them. A sermon has power in that it creates more conviction in the heart of the hearer.
As mentioned though, spiritual education needs to come from more than just a sermon. More intense study is needed. Not all of a persons theology can not be developed from just a sermon. Theological foundations built from more intense studies actually help aid the deeper understanding and appreciation of a sermon.