All adult Protestant Christians are in a state of material heresy. These Christians adhere to various Protestant heresies condemned by the Council of Trent or condemned by the Church in other ways, at other times. They obstinately doubt, or obstinately reject, important teachings on faith or morals or salvation that are required beliefs of the Catholic Christian Faith.
And yet many Protestant preachers and theologians have contributed true and useful insights into the Christian faith. For the heretic always rejects something substantial of the true Faith, but he also always retains something substantial of the true Faith. Care and caution must be exercised when learning from anyone who adheres to or teaches material heresy. For a heresy on one matter can substantially affect a teaching on another matter. All the truths of the Faith are interrelated.