What does "not allowed" mean there? I can walk into almost any Christian church in town this Sunday and not be quizzed on doctrine, nor do I know of very many churches which keep track of or somehow otherwise make sure that all of its members believe all the approved doctrines.
But as for relations between different denominations, it's usually cordial and both sides agree that the other is genuinely Christian *(even with the differences).
That would need you to be more specific.
Humans are fallen. We live in a world that is fallen and imperfect. We will always be imperfect until we reach glory. Nobody has it all right and nobody had it all wrong. We each naturally develop our own understanding of what Truth is. Not one of us has it all figured out. Problem is, we tend to lean more on human logic and formal education, while leaning less on the Holy Spirit to teach us.
If every single person could even remotely fully surrender to the Spirit's teaching as we come together in one accord, all Truth would be rightly divided because the Holy Spirit has come to help us discern Truth from lies. The Spirit is One, fully united with God the Father and Christ, the Son. He cannot be divided. Where there is division, it is never from the Holy Spirit.
The father of lies creates doubt, confusion, chaos and division. Divide and conquer is his game. Always has been and humans make themselves such easy prey.
We desire to prove we are right so much more than to be willing to see how we might be wrong. Even today, thousands of years after our Savior walked the earth, the Pharisee in all of us tends to blind us from the Truth more often than not. (I have been no exception but due to some very personal experiences in life, that is beginning to change.)
I have explored the belief systems of many denominations over my lifetime in my quest to understand why we are so divided. They all have elements of Truth that others may be missing and they all get some things very wrong. I have not found many answers I have been searching for but I have learned at least one important factor. Christians waste way to much time looking for the flaws and way too little time seeking the Holy Spirit's help to find common ground to unite upon.
Every single individual will have different ideas on what being a Christian ought to look like. Instead og trying to figure out all the texts and interpretation and theological application that even scholars with the highest degrees can't agree on, we should all be seeking the heart of Christ and asking HIM through His Spirit, in prayer to make us into His image.
We need to spend less time trying to prove and point out who is right and who is wrong and more time loving and serving as Christ did. Jesus made it simple. He took all the commandments that all the religious leaders created out of the first 10 and brought them all together in just two. Love God and love others, thinking of them as better than ourselves. Sounds pretty simple but our fallen state insists in making it so impossible to do. If we can't even unite with love for one another, we won't be able to effectively share the gospel to an unbelieving world.
They will know that we are Christians, not by how well we interpret the Bible, not how well we keep the rules, not by our wisdom or experience or how many letters we have after our name. They will know that we are Christians by how well we love. We need to get back to the simplicity of Jesus' teaching. Everything in the Bible is summed up and complete in the Gospel of Jesus Christ, and everything we need to know about how to reflect the image of God is covered in the first three words of John 3:16, "For God so loved..."
It is so simple yet people have made it so complicated to the point that wars have broke out and people have fought and been killed in the name of Christ! It is sad that most Christians can't even see the hypocrisy and absurdity of it all...yet unbelievers see it so clearly which is why they want no part of it. Oh the irony.
In the last days, when Christ calls His Church home, there will be all kinds of people from all different denominational belief systems who will find unity together. They are the Church that will be taken up. There will also be many members from differing denominations who will be left because they persued self-righteousness over God's holiness. They will remain divided and divisive. The Lord will say, 'Go away, I do not know you "
We must first seek the Kingdom and righteousness of God through Christ above all else. Otherwise we have nothing. Only then will we learn to love God and consequently one another. Only then will the Church be the Church. The Church of Christ is the body of believers UNITED in Christ. It is not a denomination and He will not allow the gates of hell to prevail against her! She is alive. She is actively growing through unity with Christ, but as long as we are caught up in a belief system and not in the Savior, we will remain divided and go down in a sinking ship called The Denomination.