Ryrie on fellowship across theological lines:
We also need to be realistic about the matter of priority in fellowship. Fellowship means sharing in common, and all areas of fellowship are not equal, simply because they do not involve the same sharing. Fellowship on the horizontal plane (that is, with other human beings) is like a series of concentric circles.
The largest circle includes all people with whom we have a certain kind of fellowship. We are to do good to all (Gal. 6:10) and to show respect in our speech to all people, believers and unbelievers, simply because all were created in the image of God (James 3:9).
The next largest circle includes all Christians. We have a certain kind of fellowship with them regardless of their affiliations or beliefs. God has done something miraculous and eternal for every person in that circle of fellowship, and we all share in common that internal divine work.
Some of the smaller circles may be our particular church fellowship or a doctrinal fellowship, such as is shared in an educational or mission affiliation. It could also be a small group or a Sunday school class, or a group of Christians serving in a specific ministry.
Cutting across all these circles is the personal factor. We obviously do not share to the same extent the fellowship we have within a given circle. Our Lord shared certain things with Peter, James, and John that He did not share with the others who were in that circle of the Twelve. As well as personal factors, there may be legitimate sociological factors that cut across the circles, and certainly geographical factors themselves limit fellowship.
The point is simply this: Circles of fellowship are not in themselves wrong; it is our failure or refusal to recognize some of them that is wrong. When someone fails to recognize the larger circles and builds a wall of doctrine or practice around the smaller one, refusing ever to move out of these circles for any reason, he is in error. Equally wrong is the attempt to make believers have the same kind of fellowship with all other believers and not allow them to have the smaller circles of fellowship.
From Charles C. Ryrie’s book, Dispensationalism, Chapter 12: A Plea