Hare Krishna
I completely agree with you that there is only one religion for all people in the world. But that is not Christianity, nor Islam, nor Hinduism, nor any other religion of this world. So, what is then that religion I speak about? That is the eternal religion of the soul IOW love of God.
Using the analogy of the sun, we can understand that the sun is not American, Indian, European, African or whatever. The sun is just a one sun for everybody without discrimination. So, same is with God, He is not African, Muslim, Christian, Indian or whatever, nor is He whatever else He is called in other universes. He is free from all material designation and thus above all quarreling of his parts and parcels - the souls that are in this material world. He is equal to all and waits that everybody turn toward Him.
So, there is one religion, the religion of the soul. And there is one God, the God for all living entities. All the living entities should therefor live like one big family by casting aside all their material designations. This is because, just as God is not Hindu, Muslim, Christian etc, all his part and parcels, the souls are also originally free from such material designations.
When one understands the importance of being free from material designation at that time one can transcend the ignorance and passions of this material world living in brotherhood with all living entities and then start the practice of the eternal religion of the soul - awakening one's love of God.
Hare Krishna
Success