Disclaimer: Though I don’t agree with everything that Leiter said, let us not miss the most important point.
Charles Leiter did a fantastic job answering this question, but I’m surprised that a Christian would even raise such a question. The Bible makes it very clear that we will know them by their fruits (Matt 7:15-20), and we will know them by the love that they have for one and another (John 13:35). Though there will never be a total ‘unity,’ there can be a total ‘love.’
Nonetheless, studying, understanding, knowing and applying the doctrines of grace are important for Christian growth. I have met some “doctrinarians” that were ‘dead’ like the Church of Sardis (Rev 3:1-6), or ‘lukewarm’ like the church of Laodicea (Rev 3:14-22), of ‘unloving’ like the church of Ephesus (Rev 2:1-8).
As Leiter stated, “I remember a time years ago, when I was around a group of guys who were all solid in doctrine, and as cold as can be.” He then added, “I remember writing in one of my notebooks, Oh God put me with men of a burning heart, not cold, carnal, doctrinarians.”
One of the formulas that I will use to test a Christians heart, is not just do they love each other within their local church (as well as universally). But do they have a passionate love for the lost (a ‘Biblical’ love without compromise). Moreover, are they able to love their enemies?
The LORD Jesus said, “By this all people will know that you are my disciples, if you have love for one another” (John 13:35).
Recently Dr. James White rebuked those of the “Calvinist club mentality,” that video can be seen here.