At the fireworks last night - rather disappointing, BTW (spoiled, I guess) - we briefly discussed Evangelical Covenant principles of agreement on the important points of the faith and freedom in the less-crucial. This stemmed from my son's recounting a question he had received at the information booth that morning - were we more Wesleyan or Calvinist in our theology?
The difficulty, of course, is that we don't all even agree on what are non-negotiable points of Christianity and which are less important. The Evangelical Covenant has some families who baptise infants, and some who hold to the importance of believer's baptism. (We have disagreement about that even in the family. Tracy and I consider infant baptism to be scriptural, Jonathan and Heidi had Emily dedicated instead of baptised. We're correct, of course, but what can you do?) A full-immersion Baptist would be unlikely to consider that a peripheral point. We take the Lord's Table only once a month in our congregation, and real presence is believed by some (us, again) and not by others. There are those who would find that looseness and lack of focus on the Sacrament to be a point too crucial to overlook in choosing a church.
CS Lewis had a paragraph or so on just this problem, with better examples. I am going to guess it is Mere Christianity, but it is evidence of my loss of grip on the Lewis corpus that I can no longer instantly tell you where it's from.
I may write a bit on what I see as the core.