Thank you for this great question. It’s one that is very meaningful for me personally because I grew up in Northern Ireland, a country that knows all too well the devastation that can be caused by religious people responding to their disagreements in a sectarian and violent way. However, my experience of Northern Ireland has also taught me that it is way too simple to claim - as many people do - that such ugly responses were exclusively the result of religion. Religious difference was only one among many other political, ideological and circumstantial factors influencing the 30 years of bloodshed that my country experienced. Many people viewed it as a war between Protestants and Catholics. The reality was much more complex!
In answering your question I think it is important to consider two issues: 1. What it truly means to be a Christian, and 2. How we treat and respond to people who believe different things
In considering the first, the bible and theologians down through the centuries have made clear that the basic requirements for anyone to be a Christian are a.) repentance and b.) personal faith in Jesus Christ as the Son of God and risen saviour. These are the ‘fundamentals’ of what it means to be Christian. So although Christianity does have a lot of “denominations”, this is the common “denominator” that makes anyone, at any time, and attending any type of church, a true Christian. That said, Christians can and do disagree on many ‘secondary issues’, such as baptism, the eucharist and the gifts of the Spirit. I say ‘secondary’ issues because, although important, what we believe about them is not fundamental to being a Christian and so believers can legitimately disagree. For some Christians, however, these ‘secondary’ issues are so important that they cannot with integrity remain in churches that believe or practice different things to their personal convictions on these matters and so they leave and start new churches or even new denominations. Of course, its not always simple to establish what is the ‘right’ interpretation to take on these matters - if it were, Christians wouldn’t disagree so much! In the New Testament we see that it took the early Church a long time and a lot of prayer and discussion to come to a conviction about what to do about circumcision and the inclusion of Gentiles in the Church. And even Peter in Matthew 16, moments after making an incredible (and correct!) confession that Jesus was the Messiah, has to be strongly rebuked by Jesus because he then makes clear that he has absolutely the wrong idea of what it means for Jesus to be God’s Messiah - he was “thinking” rightly and got this issue dead wrong even though he was a genuine believer and Apostle. So when it comes to denominations and differing opinions on secondary issues, I think we need to have a lot of grace for one another as brothers and sisters in Christ. These issues are not easy and we all have a tendency to get things wrong. No matter how convinced we our by our own convictions we need to hold them we humility and respect the difference of others.
And that leads to me the second and final point about how we treat others who believe different things than we do. Although Peter got his views about what it meant for Jesus to be Messiah very wrong in Matthew 16 and Jesus had to challenge him, he did not reject him, let alone abuse him. Jesus makes it abundantly clear that there is no justification whatsoever for attacking or defending Christianity by abusive means. Worse, in responding to people with different views by abusive means we can do serious damage to their ability to consider the truth. You see, you cannot impose truth by force, you must simply confront people with Christianity’s claims and allow them to the space and freedom to come to convictions about it for themselves. That’s how Jesus dealt with me when I believed wrong things about him. That’s how he still deals with me when, despite my best intention and efforts, I make mistakes in my understanding of him.
I trust we shall do the same with others, because our interactions with those who don’t believe Christian truth has to be as Christian in character and tone as it is in content.
I hope this helps!