This is true regardless of how much the unsaved individual and members of the body of Christ may desire to be in fellowship. The relationship is not lateral, from one believer to another, but rather vertical from each believer to the Father and Son. Jesus is the vine, we are all simply branches. New branches are grafted into the vine, not into another branch, or a group of branches. If a man is not in fellowship with the Father and Son, it is literally impossible for him to be in fellowship with Christians.
Many people mistakenly believe that the church is for everybody.
The church is the body of Christ. Unsaved people are not part of the church. The Gospel, which the church is to share, IS for everybody. Yet, most will reject it. There are two significant aspects to the Gospel.