Therefore let us, as many as are mature, have this mind; and if in anything you think otherwise, God will reveal even this to you. Nevertheless, to the degree that we have already attained, let us walk by the same rule, let us be of the same mind. (Philippians 3:15-16, NKJV).
Paul's letter to the Philippians mentions the word mind eight times. Seven of these speak of unity of mind, and one speaks of the mind of Christ which we are to emulate, and upon which our desired like-mindedness is based. Whenever we see in Scripture something repeated, it is instructive to take note. When something is repeated this many times, it is obviously very important.
In the same way that the Holy Spirit composed the Bible to anticipate all future heresies, and in the same way that no warning about falling in sin or backsliding is given that is not likely to occur, repetition is meant to clue us in that the subject being repeated needs very careful attention.
God designed intelligence, and perception, and reason, and knowledge. These things did not just happen. They are, in fact, a direct result of mankind being created in God's image. We think and make judgments because He does. We recognize patterns, and logic, and facts because He does. He is the author of all things knowable, perceivable, and reasonable. Our thoughts, while not His thoughts, are nevertheless modeled upon His. Our ability to think and form opinions, to evaluate and discover principles and corollaries, is based on His Intelligence. The adage, I think therefore I exist, is incomplete. More correctly, it is, I am made in the image of God, therefore I both think and exist.
Not only is God the author of Intelligence, He is also the God of infinite variety. He who made no two snowflakes alike, and no two fingerprints or retinal patterns alike, is the same Creator who fashioned each individual like no other who has existed, or will ever exist. At the level of essential being and personhood, there is no such thing as identical twins. Given that, it is clear that no two minds think exactly alike. While thoughts can be very similar, and while two or more people can arrive at the same conclusion, the thought processes involved in getting there often diverge quite significantly. One might leap, another might plod methodically form point a to b, and so on. And while there is much room in collective human thinking for agreement and unity, there is an equal amount of mental space in all that individual diversity for disunity.
There is no question that in Christ there is liberty and and a divine appreciation of each individual. There is more than sufficient existential room to be as expressive of that individuality as there are permutations of all the variables that make up the whole of a given personhood. So when Paul is repeatedly speaking of like-mindedness here, it cannot be defined as lock-step conformity in thought, or speech, or customs, or tradition, or dress, or anything at all that is inherent part of our uniqueness. But there must be unity on the level of doctrine and truth. There must be unity on the inward confession of Who and what we believe, and why we believe it. I can not have a view of Christ that differs from Scripture. Nor can I have my own idiosyncratic interpretation of any of the fundamental tenets of the faith. As an early church father once wrote, "…in essentials, unity; in non-essentials, liberty; in everything else, charity…".
And so we have the prophetic word confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; knowing this first, that no prophecy of Scripture is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit. (2 Peter 1:19-21, NKJV).
The term, Christian, which first appeared in the Book of Acts some 2000 years ago, means something very specific. In the course of time, that designation became diluted through overuse. C.S. Lewis gives a lengthy chronicle of this in his book, Mere Christianity, discussing the fact that when a word loses its originally intended significance, and its scope is broadened to include all sorts of things that may be tangentially related, but really beside the essential point, it usually ends up meaning pretty much nothing at all.
Like most vitally important things about faith in the Bible, the belief that saves is straightforward. It is this: Mankind since the Fall, is born in sin, and helpless to pay the penalty incurred. Jesus Christ, the Son of God, the Second Person of the Divine Trinity, came as a human being, born of a virgin, to live a sinless life as a Man. In our place, He was crucified on the Cross in propitiation (payment) for the sin of the world. He died and was buried for thee days and nights, and on the third day rose again to life. He walked among his disciples for some 40 days, teaching them that belief in Him, His life, death and resurrection was the only way of salvation. He then ascended into Heaven in front of many witnesses, to return bodily at the appointed time to re-establish the Kingdom of God on earth.
While whole volumes have been written covering the previous paragraph in great detail, that is the summary. If you agree and believe, you are rightly called a Christian. If you disagree, or disbelieve, you can call yourself whatever you want, but you will die in your sins, and suffer eternal punishment. It is this very simplicity and exclusivity of the gospel which the world finds so offensive. But that doesn't matter. If God exists as God, He has the right to set the rules. It's His game. What the game pieces think about it is utterly irrelevant.
Beyond that, there are key exhortations in the letter to Philippians, already covered, that enables a Christian to live more effectively and powerfully as a child of God. And Paul's point in these focus verses is that if you believe and begin to live as the Holy Spirit leads - putting others first, emulating Christ's example, being humble and teachable - then disagreements and disputes, when they arise, will get settled naturally.
This is true because underneath all that human individuality and diversity within the church is the Foundation, which is Christ. Submit to Him and His Word, and if in anything you think otherwise, God will reveal even this to you.
In the meantime, walk in unity of mind about the essentials, and impact the world, together, for Christ.