For example there are many people in mexico who pray to this “saint death” but many of them don’t know that it’s idoltary,they instead think that its actually part of catholic tradition. does that mean that they commited a mortal sin?
For a sin to be mortal, three conditions must be satisfied. First, it must constitute “grave matter.” Second, it must be committed with full consent. And third, it must be committed with full knowledge. A deficiency in any of these areas is sufficient to mitigate the severity of the sin.
Full knowledge is a little tricky. If one has been deceived, one is probably not acting on full knowledge – although negligence in doing one’s homework compounds the sin, rather than mitigating it. Conversely, “natural law” is intuitively known to all men, even if habitual sin has deadened their apprehension of it.
I’m not qualified to judge whether worshipping “Saint Death” qualifies as a mortal sin. Invincible ignorance is a pretty high standard – but it’s possible that illiterate, poverty-stricken peasants satisfy it. There’s no excuse for anyone with Internet access though, I’d say.