Sinful estrangement from God when a person deliberately does something gravely forbidden by God. Those who, with full deliberation and consent, commit adultery, murder, perjury, and similar crimes, estrange themselves from God. He sets down the conditions for human estrangements, not they. It is not up to a human being to decide subjectively whether a deliberate serious sin is also a mortal sin that deprives him or her of God's friendship. God along has the right to determine what separates a simmer from the Creator; a creature does not have the right to stand in judgment on god and tell him what constitutes mortal sin.
Consequently "a person sins mortally not only when his action comes from direct contempt for love for God and neighbor, but also when he consciously and freely, for whatever reason, chooses something which is seriously disordered. For in this choice there is already included contempt for the divine commandment; the person turns himself away from god and loses charity" (Paul VI, Declaration on Sexual Ethics, December 29, 1975).
Every serious sin, therefore, is a mortal sin when a person freely decides to do whatever he knows God forbids under penalty of exclusion from the kingdom of heaven.