First off, not all people who believe themselves to be Christians "are". Even though there is no question that they "BELIEVE" they are Christians, and they "Appear" to be doing exactly as other Christians "Appear" to be doing. Those who say to Jesus, "Lord, Lord ..." at the end of Matthew, for example.
The issue is the definition of a Christian. Jesus said, As you go:
1.) Make "Disciples"
2.) Baptize "Disciples"
3.) Teach "Disciples"
TO OBEY
ALL
THAT I HAVE COMMANDED YOU.
If you add to that that: "The disciples were first called Christians in Antioch." And you have the scriptural definition of a "Christian", and of a "Christian" church. If you are not doing ALL of these things, as a habit and way of life, then I question your being a Christian based on Christ's definition. If you only teach "some" of the commandments, or add and subtract some exchanging traditions and beliefs of men for the commandments of God, then, again, perhaps the "Christian" part is in question.
Does this meant that there are churches with perfect doctrines and beliefs, or even individuals with perfect doctrines and beliefs? No. But they have established biblical guidelines for dealing with the differences, they have tolerance for issues that are non-essential, and are open to publically debating and covering heresies so that the church can see and hear the truth based on all of the passages that might apply.
The question is not what our definitions of words, concepts, terms ... are. The question is, what does the word of God say and mean, and how does that fit with all that God has said, done, not said and not done ... for example, if we have created a sin that God doesn't believe is sin, and godly people in scripture do not believe is a sin, and thus it is a teaching of men being taught as a commandment of God, then Godly men will seemingly violate it, God will seem to be ignoring "sin", and will seem inconsistent, and/or God might seem to be actually participating in sin Himself, or giving commandments that seemingly allow for sin. There are many such things that are almost universal in the "Christian" churches today.