“The Church expressly distinguishes between ‘formal’ and ‘material’ heretics. A ‘formal’ heretic rejects the Church and its teaching absolutely and with full deliberation; a ‘material’ heretic rejects the Church from lack of knowledge, being influenced by false prejudice or by an anti-Catholic upbringing. St. Augustine forbids us to blame a man for being a heretic because he was born of heretical parents, provided that he does not with obstinate self-assurance shut out all better knowledge, but seeks the truth simply and loyally. Whenever the Church has such honest enquirers before her, she remembers that our Lord condemned Pharisaism but not the individual Pharisee, that He held deep and loving intercourse with Nicodemus, and allowed Himself to be invited by Simon. The spirit of the Church in her dealings with souls may be stated in St. Augustine’s words: ‘Love men, slay error!’”
-Karl Adam, The Spirit of Catholicism.