Do you think it is a sin not to be Catholic?
The following applies to those who are validly baptised…
For something to be a mortal sin (i.e., a sin unto death), there are three criteria that must be met:
(1) The behavior must be objectively evil (grave matter)
(2) The person must recognize that the behavior is objectively evil
(3) The person must make a deliberate decision to perform the behavior (full consent of the will)
For example, let’s take a look at theft of another’s property (a hopefully non-controversial issue). If you walk into a store and pocket a piece of merchandise out of that store and sneak out of the store without paying for that merchandise, the chances are you just committed a mortal sin…but not always.
(1) Is theft objectively evil? yes.
(2) Did you know that theft was objectively evil? chances are, yes (but there, in theory, might be some exceptions)
(3) Did you make a deliberate decision to do so? looks like it with the above scenario.
But, what if, as a child, your parents/guardians always covered your actions with the merchants in question so that the merchandise was paid for and you were not involved with the exchange…in theory, you might grow up thinking that whatever you wanted was yours for the taking. Under those circumstances, it is possible that you might not realize that you are supposed to pay for what you get. (Unlikely, but possible)
As to a deliberate decision of the will? What happens if your hands were full and you put something in a pocket, fully intending to pay for it at the register…but didn’t pull all of the items out of the pocket? Did you intend to steal? Or how about if you are half-crazy with hunger and had no money? Those might limit your ability to consciously exercise free will and thus would mitigate your responsibility before God for your actions.
Objectively, theft is a mortal sin…but there are potentially circumstances where your responsibility for that sin are mitigated.
So what does this have to do with your question? Well, the Church has taught, from the time of St. Paul, that there was one body of Christ, and therefore, one Church. See the discussion about followers of St. Paul vice followers of Apollos in 1 Cor 3 as an example. Causing division in the body has always been recognized as sinful (known as schism). Those who are responsible for that schism committed grave sins, without a doubt. But are those people whose fellowship is the result of that schism accountable for the sins that caused that schism?
Again, let’s look at the three criteria:
(1) Is it grave matter? Yes. (Don’t agree…look at the writings of St. Paul, particularly I Cor 3)
(2) Does the person in question recognize it to be grave matter? Well, if he did, then would he still be involved with the schismatic group? Doubtful.
(3) Did the person involved make a deliberate decision to be schismatic? Again, it likely was a result of the fact that his parents, and their parents, for many generations were involved with schismatic groups.
The big exception is (potentially) those who were baptized and catechised as Catholics and then left the Church. Those individuals,
if they were fully catechised on the subject (which is a big
if, especially with the current state of catechesis), are in a different situation than those who were never Catholic to begin with.
A long answer, I realize, but hopefully it gives you what you need.
It’s in the Bible.