To review, there are two kinds of actual sins which we commit: mortal and venial.
Mortal sins kill the life of grace, sanctifying grace, in the soul. Mortal sins must be of serious matter, committed with knowledge of their serious nature and with consent of the will. They deserve the eternal punish of separation of God which is Hell.
Venial sins are lesser offences against God and His laws. They do not deprive the soul of sanctifying grace. If the matter is not grave or when one doesn’t fully grasp how serious it is, or if consent of will is lacking, the sin can be venial rather than mortal. Venial sins can be forgiven through certain works and in moments of the Mass, etc.
Let’s be clear about something: venial sins are SINS. They offend God, Mary and the saints and angels. They endanger our souls because they can lead to worse sins or the deadening of our minds and hearts about the true nature of sin’s horror. Moreover, as one goes more deeply into the spiritual life and successfully roots out serious faults, then the smaller sins take on more importance. St Teresa of Avila says that the first great obstacle she had to overcome in advancement of her spiritual life was carelessness about sin.
