If I recall correctly Alexander Campbell was a Baptist minister. He developed a soteriology regarding baptism that would be rejected by Baptists. Thus, the 'Campbellites' had to separate from the Baptists. Now suppose instead the Baptists and Campbellites 'joined together'. They certainly could agree on many things, including the importance of baptism, and even that baptism is a necessary commandment of the Lord that is to be obeyed. But their union would never last, because the Campbellites believed baptism is the event in which remission of sins is received, and further that one being baptised must believe they are being baptised IN ORDER TO receive remission of sins. Baptists of course hold to a different view, that baptism is the public testimony that one HAS RECEIVED ALREADY the remission of their sins.
The two could never continue in unity. In fact they didn't. The Campbellites separated and today they are known as the Churches of Christ, the Disciples of Christ, and the Christian Churches (names of their non-denominational denominations).
The union of the PCI and the PAJC has lasted longer than the Campbellites and the Baptists (much longer, in fact), even though the differences are parallel between the two groups of movements.
Perhaps it would be better for the UPCI be united on matters of doctrine, not on matters of organizational or missional purpose.
And I'm stil linterested in hearing a response to my question:
Should Christians pursue doctrinal, Biblical unity? Or should we be content to find the lowest common (acceptable) denominator, and base our unity on that? (Keeping in mind the injunction of
Ephesians 4:1-13 to keep the unity of the Spirit until we come into the unity of the faith.)