We profess at each Mass, our belief in a church that is one, holy, catholic, and apostolic.
What does it mean to say our church is "one"?
There was one church established by Christ. When He tells Peter, "you are rock, and on this rock I will build my church", Jesus speaks in the singular. He establishes a single church against which "the ... ... shall not prevail."
We are one Body of Christ. We are the body and Jesus is the head of the Church; a head cannot belong to multiple bodies. We are made in God's image in many ways and this is one; a single head leads a single body.
Fianlly, Jesus is the Way, the Truth, and the Life - all singular. There is one truth. Other systems of belief may have some of the story, they may have some of the truth and the glory of God. They do not have Christ at their head. Christ founded one church on one truth. That means there can be only one completely correct answer - one church that is right. (God, in His great love - and as a generous teacher - gives a lot of partial credit.)