The Christian concept of internal sin is the idea that sin

Sin

In religion, sin is the violation or deviation of an eternal divine law or standard. The term sin may also refer to the state of having committed such a violation. Christians believe the moral code of conduct is decreed by God In religion, sin (also called peccancy) is the violation or deviation...

 (a crime of religion) 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

Heresy

Heresy is a controversial or novel change to a system of beliefs, especially a religion, that conflicts with established dogma. It is distinct from apostasy, which is the formal denunciation of one's religion, principles or cause, and blasphemy, which is irreverence toward religion...

, but the Reformation

Protestant Reformation

The Protestant Reformation was a 16th-century split within Western Christianity initiated by Martin Luther, John Calvin and other early Protestants. The efforts of the self-described "reformers", who objected to the doctrines, rituals and ecclesiastical structure of the Roman Catholic Church, led...

's alarms received new emphasis at the Council of Trent

Council of Trent

The Council of Trent was the 16th-century Ecumenical Council of the Roman Catholic Church. It is considered to be one of the Church's most important councils. It convened in Trent between December 13, 1545, and December 4, 1563 in twenty-five sessions for three periods...

 (Session XIV, chapter. v). The session, while reiterating that all mortal sin

Mortal sin

Mortal sins are in the theology of some, but not all Christian denominations wrongful acts that condemn a person to Hell after death. These sins are considered "mortal" because they constitute a rupture in a person's link to God's saving grace: the person's soul becomes "dead", not merely weakened...

s 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

Catholicism

Catholicism is a broad term for the body of the Catholic faith, its theologies and doctrines, its liturgical, ethical, spiritual, and behavioral characteristics, as well as a religious people as a whole....

:

• delectatio morosa, i.e. the pleasure taken in a sinful thought or imagination even without desiring it;
• gaudium, i.e. dwelling with complacency on sins already committed;
• desiderium, i.e. the desire for what is sinful.

Biblical quotes

• Matthew
  Gospel of Matthew
  The Gospel According to Matthew is one of the four canonical gospels, one of the three synoptic gospels, and the first book of the New Testament. It tells of the life, ministry, death, and resurrection of Jesus of Nazareth...
  
   5:28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
• Romans
  Epistle to the Romans
  The Epistle of Paul to the Romans, often shortened to Romans, is the sixth book in the New Testament. Biblical scholars agree that it was composed by the Apostle Paul to explain that Salvation is offered through the Gospel of Jesus Christ...
  
  6:12 Therefore do not let sin reign in your mortal body so that you obey its evil desires.