Pope Leo XIII on the Worst Kind of Heretic

The worst kind of heretic is the one who, while teaching mostly true Catholic doctrine, add a word of heresy, like a drop of poison in a cup of water.

— Pope Leo XIII

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Prayer to St. Irenaeus Against Heresy and Unfaithfulness 

Ever living God, Our Father, You granted that Blessed Saint Irenaeus, Your martyr and bishop, should overcome heresy by the truth of his doctrine, and establish peace in Your Church. Give to Your people, we entreat You, constancy in their holy religion. Help us defeat the Gnostics of our day, all heresy in any form, and grant us peace in our time. This we pray through Jesus Christ our Lord. Amen.