Denomination doesn’t matter, if you’re working from Protestant presuppositions, because the nature of the church, in their view, is such that it is not present in a “fullness” in a visible entity.
Protestantism is complicated and diverse, and so this is just a general sketch. After all, even some of the original Reformers would damn you if you were not part of their particular group or teaching. Look up Calvin’s own tolerance, for starters.
But the Catholic does not work with these assumptions. The Catholic view is prior and equivalent to early Christianity, which is maintained by the various Eastern traditions as well: That the church subsists in a visible entity that is held together by concrete, sacramental realities, like Apostolic Succession, a continuous liturgy, the maintenance of Christian teaching.
The reason a Catholic would say “denomination” matters is because (1) Truth matters. If a Christian group says baptism is NOT regenerative, well that’s error, and a serious one. Is the Eucharist only symbolic? That’s a serious question. Etc. (2) the Church IS identifiable with a visible entity — and not a mere invisible collection of regenerated souls, wherever they may be (Baptist over here, Catholic here, Evangelical here, etc.)
But I think people often miss the reality that Catholic teaching implies that Protestants PARTICIPATE in the one Catholic Church. They do not do this by their denomination in itself, but with those elements they have maintained from Catholicism. Baptism, so long as it is valid, REALLY DOES make the Protestant Christian part of the Body of Christ. The Protestant Bible, though incomplete, REALLY DOES contain the Word of God. Wherever two or three are gathered in Christ’s name, CHRIST IS REALLY there.
But the Catholic (or any other ancient Christian) is not content with this. There is no such thing as “mere Christianity.” There is a Church founded by Christ on the rock of Peter, which has been led through apostolic succession since day one.
And it is important that ALL Christians get closer and closer to this one source of Catholic communion, because then they will get closer and closer to and more and more of the fullness of Christian teaching and means of sanctification.