The Catholic Church was founded by Christ to be the pillar and foundation of all Truth. He gave us this truth to be a visible authority over time, so that we might be instructed in all things as he instructed the Apostles.
That being said, yes, if one was a Catholic, and understands this truth, that the Catholic Church teaches God’s truth so that me might be saved, and then, despite this realization, one chooses to turn ones back on that Truth and the Church that teaches it, yes, that would be a sin.
There are however, many Catholics who never had a full understanding of the gift they were given in their membership in the Church. They may have been poorly catechised or they may simply have not understood what was put before them (sometimes sin seperates us from truth, ultimately though, Faith is a gift of Grace and we need to be willing to accept the gifts we are given, they are not shoved down our throats, even if parents might try to do so).
So truly, in the end, only God knows the true disposition of the soul that leaves the Church. And it is this disposition of the soul that makes for a sin.
When one turns away from God, from having our Lord as our God, this is a sin. It can be either a Major sin (we call it a Mortal Sin) or it can be a minor sin (we call it venial), but either way, turning from God does damage to our soul and that’s really what sin is, a seperation that we create between us and God, our willingness to cease doing the will of God and then taking action on our will.
It is also my understanding that one can formally leave the Catholic Church by informing their local Bishop of their intent to do so. This would relieve you of the Canon Law obligation to follow the teachings of the Church, but it would not change either Natural Law or Moral Law, both of which, as taught by the Church, but not being reserved to the Church, have been given to us for our own good. Living in accord with the teachings of the Church will allow us to live our lives most fully and hopefully with fewer errors to regret.
What you are is a lapsed Catholic. I’ll pray that one day God will lead you back to His church. God bless you.
That is your interpretation of V2, which some other RCs differs with, and no, i am not one whose dissent from Rome is due to ignorance, or who, “knowing that the Catholic Church was made necessary by Christ,... refuse either to enter it or to remain in it, (CCC #846) but instead i know quite well her claims that it is the Roman Catholic church of today that is the OTC, though now she broadly affirms Protestants as saved, but i “know” from Scripture that the the claim of Rome is presumptuous.
And thus i was/am constrained by conscience and the assured Word of God to give assent to the Scriptures over her claims, wherein they differ, as well as defend those we both concur with.
And,
“Over the pope as the expression of the binding claim of ecclesiastical authority there still stands one’s own conscience, which must be obeyed before all else, if necessary even against the requirement of ecclesiastical authority. Conscience confronts [the individual] with a supreme and ultimate tribunal, and one which in the last resort is beyond the claim of external social groups, even of the official church” (Pope Benedict XVI [then Archbishop Joseph Ratzinger], Commentary on the Documents of Vatican II, ed. Vorgrimler, 1968, on Gaudium et spes, part 1,chapter 1.).
You can protest that my choice means i am relying on my fallible human reasoning, however prayerful, and have need of an assuredly infallible interpreter, however most of Scripture was established and truth was preserved before there was a papal claim by Rome.
In addition, you yourself made a fallible decision to join or remain in the RC system and exercise the same in interpreting her.
Even as regards infallible teaching, RCs own “internal assent is obligatory only on those who can give it consistently with the claims of objective truth on their conscience, it is assumed, being directed by a spirit of generous loyalty to genuine Catholic principles [which again is a fallible choice.]
But before being bound to give such an assent, the believer has a right to be certain that the teaching in question is definitive (since only definitive teaching is infallible); — http://www.newadvent.org/cathen/07790a.htm
And deciding this involves your own finite interpretation in many cases, as there is no infallible canon of all infallible teaching, and thus there is variance among RCs as to how many there are, with only a few being accepted as such out of potentially multitudes. And reliance upon finite interpretation is also the case to differing degrees in all that RCs hold to and practice.
Thus we both made a potentially fallible choice to hold to our respective assuredly infallible supreme authorities, Scripture and the sacred magisterium respectively, which we both assent to and interpret, though lacking assuredly infallible interpretation of either.
Both parties (sola scriptura and sola ecclesia) typically hold to some core teachings while suffering varying degrees of interpretation on other things (and RCs can and do differ in more than they realize), the difference only being in matter of degrees.
And not that SS does not disallow the magisterium or tradition as helps in understanding, nor the judicial authority of the former, but that neither is assuredly infallible, as in Scripture truth was not based upon a perpetual assuredly infallible magisterium of men - which idea is not supported but wrested from Scripture - but instead truth claims were established upon Scriptural warrant, both textual and by the manner of supernatural attestation it provides for, and under which it first was penned. To the glory of God.
“For the kingdom of God is not in word [self-proclamation], but in power,” (1 Corinthians 4:20) first and foremost by the evangelical gospel which convicts and converts souls with manifest regeneration, often in the same hour as they first heard the good news (though the prep work can take decades).