In essentials, unity; in nonessentials, liberty; in all things, charity. - Rupertus Meldenius.
This idea is spoken of in many ways, but they all have the same big idea: there is a core of Christian theology which is essential and must be upheld without compromise, and a periphery for which diversity is allowed and even encouraged.
While there is considerable debate over which doctrines should be really considered essential, there is also a broad consensus for some central doctrines. Numerically, most Christians (Trinitarians) consider the Nicene Creed to be the most important doctrinal statement that unites true Christians and separates them from the heretical churches who cannot uphold the entirety of the Creed. Conversely, many non-Trinitarians (and the non-Nicene Christians who consider themselves Trinitarian but differ on some details) would consider the theology of the Nicene Creed to be heretical, and so it marks out a boundary respected by most people on both sides of it.
On the other hand, there are issues which pretty much no one would consider to be essential; things like musical or liturgical styles, church governance models (like whether you have one leader at each level, such as a pastor, priest, bishop, etc., or a plurality of equal elders), or whether a small group that meets to study the Bible and pray together should be called a Bible Study Group, a Home Group, a Cell Group, a Community Group, a Gospel Community, a Life Group, etc...
But there are things that most of us would say are far more important than the trivialities (what colour carpet to install) but are not essential either. It's helpful to have a third category: things that are necessary for cooperation in Christian ministry and mission. For example, Protestants say that there are many faithful Christians in the Catholic and Orthodox Churches; they don't think that only Protestants are saved or only Protestants are True Christians. But the doctrines of the Five Solas are considered very important to Protestants, as they concern our understanding of the Gospel and refute several distortions of it. Because of these differences, Protestants and Catholics usually don't cooperate in evangelistic projects; when a core task of evangelism is to explain the Gospel, it's hard to work together at that task with people who think of the Gospel in very different and sometimes contradictory ways to you. (This doesn't mean that Protestants and Catholics can never work together; for example, they often sponsor Bible translation projects together.) Similarly, those who support women's ordination and those who oppose it would likely have many issues if they tried to work together in a church planting organisation. And this is also why almost all denominations pick a side in the infant baptism debate rather than being deliberately agnostic about it; as a sacrament, baptism is one of the most important things of the Church, and most Christians consider it important for churches in their denomination to have a consistent position on the sacraments even if they differ about many other things.