Internal sin

Internal sin, in Christianity, is the idea that sin may be committed not only by outward deeds but also by the inner activity of the mind, quite apart from any external manifestation. Thought crimes were as old as heresy, but the Reformation's alarms received new emphasis at the Council of Trent (Session XIV, chapter. v). The session, while reiterating that all mortal sins must be confessed, singled out the unspoken ones that "sometimes more grievously wound the soul and are more dangerous than sins which are openly committed".
Three kinds of internal sin are usually distinguished by Catholics: