Sin is the free transgression of a divine law, and can be further defined as any thought, word, or deed against the Law of God. Sin may be mortal or venial.
It is mortal when the transgression is of a divine law in a matter that is serious and when the consent to sin is made with the recognition of both the law and the serious matter. It is called mortal since it deprives the soul of its supernatural life of sanctifying grace, and it deserves eternal punishment, since the offence is a deliberate act of rebellion against the infinite majesty of God.
According to the Catechism of the Catholic Church, for a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent” (1857).
A sin is venial when it is committed out of imperfect knowledge and consent, when one transgresses law that does not bind seriously, or when a sin is actually grave but, because of an erroneous conscience, the one committing it is ignorant of it gravity.
This sin does not deprive the soul of sanctifying grace. Venial sins can be remitted by prayer or other good works.
Sin is also classified according to type, that is, internal sins are those committed through the use of the spiritual faculties, for example, imagination; actual sin is any act or omission of a prescribed good act; and, habitual sin is a state of sin of one who has not repented.
The Church has always taught that every sin, no matter how serious, can be forgiven.
Sin is present whenever one tries to separate themselves from God and ceases to acknowledge one’s dependence upon God. Because of this, pride is consider the basic or fundamental sin, because it is an action that brings about a denial of the faithfulness of God. (9:51)