Catholic apologists make the following assumptions:
i) There is one true church
ii) All Christians ought to belong to the one true church
iii) The Roman church is the one true church
Admittedly, this is less clear-cut than it used to be. Vatican II took a more ecumenical approach. Rome no longer wants to say Protestants or Orthodox Christians are "outside" the church. So the relationship is parsed in terms of the "fullness" of grace and truth in the Roman church, or variations thereof.
It's striking that the NT never says there is "one church". The NT uses the metaphor of the temple. The significance of this metaphor is that what makes a building a temple is the divine presence within the temple. As Paul adapts this metaphor, a Christian is a temple of the Holy Spirit.
That, however, means whoever has the Spirit ipso facto belongs to the church.
Another corporate metaphor is a flock of sheep. That isn't called the "church". Rather, it's a collective metaphor for Christians. But if we use it as a synonym for the church, then whoever has Jesus as their shepherd belongs to the church.
Finally, the Paul uses the "body" as a metaphor for the church. And he says there is "one body." That's the closest you get to a "one church" formula in the NT.
If there's one body, and the body is a synonym for the church, doesn't that mean there's one church?
i) In a sense. However, this is a flexible metaphor which Paul uses to illustrate diversity as well as unity or unicity. He alternates between the one and the many.
ii) In addition, Paul says:
so we, though many, are one body in Christ, and individually members one of another (Rom 12:5).
To be in Christ makes you a member of his body. That's the constitutive relationship. That's what makes all believers belong to one and the same body.Christians have a corporate identity by virtue of their incorporation in Christ.
But in that event, each and every Christian already belongs to the "one church" in virtue of their union with Christ.