Now I urge you, brethren, keep your eye on those who cause dissensions and hindrances contrary to the teaching which you learned, and turn away from them.Romans 16:17
Christians use this passage to disfellowship people who disagree with them by claiming that those who teach something other than them are eligible for being “marked” and avoided. What's ironic is this passage is actually addressing individuals who cause unnecessary divisions over matters of opinion.
The “teaching” Paul has in mind is not acappella singing, communion every Sunday, or one communion cup for the grape juice; instead, he is talking about his teaching he introduced in Romans 14.
The word “hindrances” is from the Greek word σκάνδαλον (skandalon). Paul uses the same word in Romans 14:13.
Therefore let us not judge one another anymore, but rather determine this—not to put an obstacle or a stumbling block in a brother's way.Romans 14:13
These individuals are who Paul had in mind when he said to keep an eye on those who cause divisions. People who add to the New Covenant by inventing restrictions that the New Testament speaks nothing of our the very ones Paul warned the church at Rome about.
So, what is the standard for those in Christ? How do we determine if someone is a fellow believer?
Well, it is quite simple.
John said, “The one who says, “I have come to know Him,” and does not keep His commandments, is a liar, and the truth is not in him” (1 John 2:4). I heard one preacher say, “Baptists are liars! Methodists are liars! Lutherans are liars!…” and on and on and on. He interpreted this passage as anyone who disagrees with him about the Bible. If someone worships with instruments, takes the Lord's Supper infrequently, or disagrees with some other specific doctrine that the New Testament doesn't address, they are a liar in his eyes.
But this isn't what John is talking about.
When we read just a little more into the book, John explains what the commandment are:
This is His commandment, that we believe in the name of His Son Jesus Christ, and love one another, just as He commanded us. The one who keeps His commandments abides in Him, and He in him. We know by this that He abides in us, by the spirit whom He has given us.1 John 3:23–24
If you can find someone who believes in Jesus and loves their neighbor, then you have found a brother or sister in Christ. Keep an eye on individuals who cause divisions concerning issues other than these.
For an extended study on this passage and others that are commonly misused, see Dr. Dallas Burdette's Book Old Texts through New Eyes: https://freedominchrist.net/dallass-books/.