I think that indeed there is only one Christian church. That "church" is the community of believers who believe in the Word of God and is based on the redemptive work of Jesus. It is made up of the people who have experienced God's love and have the love of God living within them. This "church" comes about by the working of the Holy Spirit. The "church" consists of people from all denominations, beliefs and situations.
The various denominations are not the "church" they are creations of man. They are all flawed and imperfect. An unfortunate result of the Reformation was that it gave people the ability an incentive to create churches that suited themselves. Anyone can create an organization and call it a church. The organized churches may contain many members of the universal "church" of believers, but they are not the "church" itself.
For I am convinced that neither life nor death...neither the present nor the future nor anything in all creation will be able to separate us from the love of God that is in Christ Jesus our Lord. Romans 8:38-39