Christians are called to unity. (John 17:21-23, Romans 12:5, 1 Corinthians 1:10, etc.). Unity requires harmony, like-mindedness, having things in common. However, until we are perfected in glory, there is an inverse relationship between the number of people with whom we have unity and the number of things about which we are unified. For example, if I state that “it’s good to be happy,” this is something that almost everyone can agree on. However, as soon as I add to that statement (for example, “watching the St. Louis Cardinals win makes me happy”), the size of the group that is in agreement will shrink.
So, in our pursuit of unity, there is a balance between pursuing unity with the maximum number of other Christians possible and pursuing unity in as many areas as possible. In the new heavens and new earth, there will be 100% agreement over all things with all people. In our current fallen world, we sometimes have to choose between limited unity with more people or more unity with fewer people.
If you take the route of pursuing unity with the maximum number of other Christians, you will find that the number of things about which you are unified must be relatively small. This can lead to a pretty anemic version of Christianity, where “doctrine” consists of little more than platitudes.
On the other hand, trying to achieve unity in as many areas as possible can lead to isolation. One of the connotations of “fundamentalism” is the idea that you only associate with those who share your opinions on just about everything. When you can’t come to agreement on an issue, you splinter off into smaller and smaller groups.
A healthy approach is not simply to find the “midpoint” where you have a reasonable level of unity with a reasonable group of believers, but to have a balance of both types of unity: a mix of basic unity with the larger body of Christ and a smaller group of believers with whom you can enjoy deeper unity on more levels. If all of your unity with other believers is at a very basic level, you’re probably lacking in doctrinal depth and missing out on the joy of pursuing and discussing rich theology with others who are likeminded. If all your unity with other believers is with a small group of others who agree with you on almost all matters, you’re probably missing out on the diversity within the body of Christ and you’re probably not being sufficiently challenged to love, respect, and work together with those who hold different convictions than you.