Similar back and forth. I grew up in an independent pentecostal church, became a Catholic for a number of years and are now back in pentecostalism (never really left). The imagery of a bulls eye is the best one can relate to. The bulls-eye represents the fundamental core doctrine that defines the body of Christ - the Person, Nature and Work of God, Christ and the Holy Spirit, Salvation. The next ring are other doctrines that define the denominations with the outer ring being the various doctrines that are not essential to salvation. Paul related the Body of Christ (the universal Church) as a body - some were arms, others feet, etc with Christ as the head. So just as people fill the parts of the body in the local church, churches (denominations) fill parts of the universal body.
If there were serious doctrinal differences, especially about salvation, among these denominations as was stated by your opponent, then how could lay as well as clergy members of all these supposed “opposing” faiths participate in sharing the same message with the blessings and support of their churches and organizations?
The fact is - except for some of the radical fringes - they would not be able to as you correctly point out.
Good analogy. A Catholic Priest I work with from time to time and I had a discussion along similar lines. Basically the difference are liturgical and in method. We all have different procedure manuals and facilities, but our product is the same.
*** The imagery of a bulls eye is the best one can relate to. The bulls-eye represents the fundamental core doctrine that defines the body of Christ - the Person, Nature and Work of God, Christ and the Holy Spirit, Salvation.***
That is the same example I use.