Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act and its opposition to God's law. It also requires a consent sufficiently deliberate to be a personal choice. Ignorance, through no fault of the individual (referred to as invincible ignorance by the Church), can diminish or even remove the imputability of a grave offense.
We are all born into sin a fallen state that needs redeption and that redeption only comes from Christ.