Here is an interesting article (old but informative) by Bro. Peter Dimond of MHFM discussing the Catholic principle on when we must avoid heretics in the act of receiving sacraments from them and the distinctions between a heretic according to the Divine law and a heretic according to Church law. It discusses when a heretic is considered tolerated (before a declaration or notoriety of law or fact comes into play) or when he becomes declared or notorious in law or fact and thus to be officially avoided by the faithful. I encourage everyone on here to give this a thorough read.