Today's Christians are more divided and divisive than ever. During the first centuries of the history of the church it was basically universal or catholic until the 1054 Schism and the Reformation of 1517. There is now a sect or denomination for every niche or group and doctrine. Some have a fetish with a favorite doctrine much like the Pharisees did with the Sabbath command. When Paul wrote to the Corinthians, he chided them for saying and grouping into factions: I am of Paul, I am of Cephas, I am of Apollos, or even I am of Jesus (as if the latter trumped all others and its followers were especially holy or righteous).
CAVEAT: when you think your denomination or sect has a monopoly on truth or you cannot get along or fellowship with "outsiders" or other faiths, your church is on the verge of being a cult, especially when you think you are the only ones and see your mission statement as one of proselytizing, not evangelizing.
Does it really matter to you what a believer relates to, Paul is admonishing us to not divide or judge another by their affiliations. I'm not saying all Christians should forget doctrine and just get together and sing kumbaya. Even Paul separated from Barnabas and had a dispute and Peter from Mark but I believe they were not petty or nitpicking on doctrine or splitting hairs. But Paul didn't preach about why he did it nor try to justify it.
God condemns us for being divisive and especially for causing division between brothers or disharmony or discord. That is one of the sins that God hates as mentioned in Proverbs. Satan likes to divide and conquer! We are not to be ignorant of his schemes and wiles. We are to leave room and space for people and make allowance for differences of opinion and disputable matters. But each should be convinced fully in their own minds and have a clear conscience.
What is God concerned with? He will not ask us at the Judgment Seat what kind of church we went or whether we were this or that type of theological school of thought at all. Did we love our brother? We will be measured as to our righteousness, faithfulness, obedience, and love, and our walk with the Lord. And the works done in the flesh will amount to nil. We are accountable for our orthopraxy or ethics as much as our orthodoxy of doctrine.
What doctrine is meant to do is strengthen our faith, not divide us! And we are rewarded according to our works, not our faith, which is a gift of God. Only what's done for Christ in His name and to His glory in the Spirit can be rewarded and last.
There will be no appraisal of the orthodoxy of our doctrines except the basic ones (we are to contend for the common faith we share as Christians). We are not commended then because we were impeccably right in our doctrines. Doctrine is important for faith to grow and we are to resist heresy and expose it, but there are many disputable ones that sincere and strong believers can disagree on. We shouldn't wage war against them because they disagree with us nor be on a mission or agenda to make everyone believe as we do. We must rather practice what we believe and put our faith into action, turning creeds into deeds.
Therefore, someone is not a good Christian because he doesn't or does affiliate with some denomination or identify with a theological interpretation such as Calvinism which is so controversial now that they renamed themselves "Reformed" or "Covenant." This is like many churches including mine that have taken the name "Baptist" off and renamed it some non-denominational sounding one. Some so-called Evangelical churches are close to what my church stands for and we do not promote unique doctrines in order not to divide the brethren.
But Baptists get a bad rap and some people associate them with backsliders, predestination, legalism, or eternal security and therefore people already have a preconceived notion of any believer who belongs to such a church. Did you know that Billy Graham was a Southern Baptist but he didn't propagate or preach it in his evangelism. He learned to cooperate even with Catholics in evangelistic outreaches. For sure, note that God will not ask you what church you attended but whether you found a way to serve and were faithful.
Now, the question arises as to our fellowship with others. We are not only to be in fellowship with our church family but strive to be at peace with all men and love the brotherhood everywhere. Jesus said we would be known by our love, not our branding. It really doesn't matter what you label yourself, you are not holier for doing it and God condemns any "holier than thou" attitude. We are to observe the weightier matters of the Law like "justice, mercy, and faithfulness."
Anyone can call themselves a Christian but some are nominal believers or Christians in name only. The church is really composed of all true believers worldwide, not just our own fellowship or circle of friends and family. If someone is of another sect or denomination or school of theology, it should not matter to you; to his own Lord he stands or falls. We should determine in the end who serves God and knows God and who doesn't. We must heed Christ's words that they will know we are Christians by our love for one another.
In the final analysis, we ought to celebrate our common faith and commonalities, not resent differences. Fellowship happens when we stop judging or even labeling each other and commence accepting and loving one another in the Lord. Soli Deo Gloria!