How much ground to cover? The limits of sermon content

I went to church at Metro Church of Christ today and enjoyed worshipping with the group there. I had the opportunity to share a little about what Cascade Hills was doing in Salem (and help a few folks do a little fundraising while they were at it).

While there, the minister preached a sermon about covenant and what our covenant with God has to say about who we are and what's expected of us as Christians. The sermon was located in Deuteronomy 7, highlighting a few points about Israel's relationship to God following their covenant-making with him at Mt. Sinai. The sermon covered a lot of the waterfront about what covenants are, their background, and how a covenant with God is both different from and similar to the contracts we make in other areas of life. The point of the sermon was that we have responsibility to live up to our end of the covenant, regardless of the fact that God will always be faithful to his end. Good sermon...

I did notice one thing though: in the process of preaching about this (and because he was focusing on the question of our responsibility within covenant), the preacher bumped into several huge topics: that of election, Calvinism, and related topics. With as much as he had to cover with his main subject, he didn't have time to spend on either of these others. During the course of the sermon, he bumped into a few question-generating areas and was made a few wide gestures in an attempt to answer them, making several quick references to Calvinism and the doctrine of election.

Trying to listen to the sermon as a fellow preacher, my mind was swirling with the potential pitfalls of opening up these cans of worms in the midst of sermon without the time or resources to adequately deal with them. No doubt some members knew exactly what he was talking about in these other areas and the brief references were enough. But what about those who didn't have enough background to catch them?

This has become a major question for me: as one who engages in conversation around Christian ideas with others, how can one say much of anything without having to explain dozens of other equally complex and difficult issues along the way?

At the risk of a grass-is-always-greener sort of comment, I wonder if it was easier when the average person had a better grasp of basic biblical concepts. Phew, where do you start?