Hi Singer,
As C.S. Lewis said, "Words mean things," and once we take away the meaning of a word or redefine that meaning, then there is no use in still using that word.
You wrote, "On the other hand, catholicism (universal body of believers) could be considered the trunk."
The renowned Protestant historian J. N. D. Kelly writes: "As regards ‘Catholic’ . . . in the latter half of the second century at latest, we find it conveying the suggestion that the Catholic is the true Church as distinct from heretical congregations (cf., e.g., Muratorian Canon). . . . What these early Fathers were envisaging was almost always the empirical, visible society; they had little or no inkling of the distinction which was later to become important between a visible and an invisible Church" (Early Christian Doctrines, 190–1).
The word, from the beginning of its usage, has meant to signify the visible Catholic Church as distinct from heretical congregations.
To redefine "Catholic" as "all true believers irregardless of the sect they fall within" is essentially to no longer regard "Catholic" as a word or to alter its historic definition.
Why are you intent on doing so? Why not allow for Catholics to call themselves such and for you to call yourself a non-Catholic, remaining faithful to the historic usage of the title?
Secular sources are clear that the Roman Catholic Church is the self-same Catholic Church as spoken of by the Church Fathers. For example:
"By A. D. 100, ... Christianity had become an institution headed by a three-rank hierarchy of bishops, priests, and deacons, who understood themselves to be the guardians of the only 'true faith'. The majority of churches, among which the church of Rome took a leading role, rejected all other viewpoints as heresy. Deploring the diversity of the earlier movement, Bishop Irenaeus and his followers insisted that there could be only one church, and outside of that church, he declared, "there is no salvation." Members of this church alone are orthodox (literally, "straight-thinking") Christians. And, he claimed, this church must be catholic-- that is, universal." (The Gnostic Gospels by Elaine Pagels {Vintage Books 1994})