You must make a distinction between "grave matter" (objective) and "mortal sin" (something you knew and intended in a deliberate sense; what you are subjectively answerable for).
[Vatican II Lumen Gentium, para 14]
"He [Jesus Christ] Himself explicitly asserted the necessity of faith and Baptism, and thereby affirmed at the same time the necessity of the Church which men enter through Baptism as through a door. Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it."
“Perhaps you didn’t read what I sent a whole back at #5141 To summarize, to fail to join, or fail to remain in, the Catholic Church, is grave matter, since God wills us to be members of the Church he built, under Peter the Chief Shepherd and his successors. However it cannot be said to be a mortal sin if you are ignorant of the fact that God wills you to be in this Church.”
MDO,
I realize you believe this from your catholic upbringing or catechism. However, hundreds of millions of Christian believers do not believe:
That the Catholic denomination is the “church He built.”
That it was built under or upon Peter.
That there was an unbroken chain beneath Peter.
That there is to be one earthly central authority over all churches, like McDonalds franchisees under a corporate headquarters.
And finally, we do not believe it is a mortal sin to not be Catholic. We wish that every person, including every Catholic would come to saving faith in Christ. He alone is over every local gathering.
Best.