The Wikipedia article on mortal sin has a couple of interesting tidbit (and it actually gives their sources!).
**Though not part of the dogma of the Orthodox Church the mortal/venial distinction is assumed by some Orthodox authors and saints as a theologoumenon. For example, Saint Ignatius Brianchaninov (1807–1867), in his book A Word on Death, in a chapter entitled “Mortal sin”, says:
It has been said earlier that mortal sin of an Orthodox Christian, not being cured by repentance, submits him to eternal suffering; it has also been said that the unbelievers, Muslims, and other non-orthodox, even here are the possession of hell, and are deprived of any hope of salvation, being deprived of Christ, the only means of salvation. Mortal sins for Christians are the next: heresy, schism, blasphemy, apostasy, witchery, despair, suicide, fornication, adultery, unnatural carnal sins,* incest, drunkenness, sacrilege, murder, theft, robbery, and every cruel and brutal injury. Only one of these sins—suicide—cannot be healed by repentance, and every one of them slays the soul and makes the soul incapable of eternal bliss, until he/she cleans himself/herself with due repentance. If a man falls but once in any of these sins, he dies by soul: For whosoever shall keep the whole law, and yet offend in one, he is guilty of all. For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law. (James 2:10,11)
- Under “unnatural carnal sins” the following are implied: sodomy, bestiality, masturbation, and any unnatural intercourse between married people (such as using contraceptives, consummated oral or consummated anal intercourse, etc.) as is explained in the book Ascetical Trials, also written by Saint Ignatius Brianchaninov (1807–1867).
Similarly, the Exomologetarion of Nicodemus the Hagiorite[37] (1749–1809) distinguishes seven classes of sin:[38]
Pardonable
Near the pardonable
Non-mortal
Near the non-mortal
Between the mortal and the non-mortal
Near the mortal
Mortal
Nicodemus gives the following example for the seven classes of sin. “The initial movement of anger is pardonable; near to the pardonable is for someone to say harsh words and get hot-tempered. A non-mortal sin is to swear; near the non-mortal is for someone to strike with the hand. Between the non-mortal and the mortal is to strike with a small stick; near the mortal is to strike with a large stick, or with a knife, but not in the area of the head. A mortal sin is to murder. A similar pattern applies to the other sins. Wherefore, those sins nearer to the pardonable end are penanced lighter, while those nearer to the mortal end are more severely penanced.”
He also stipulates seven conditions of sin:[39]
Who is the doer of the sin
What sin was committed
Why was it committed
In what manner was it committed
At what time/age was it committed
Where was it committed
How many times was it committed**
So apparently there is a (relatively) recent tradition in the Byzantine East of making the mortal/venial sin distinction. And that tradition posited by none other than one of the greatest saints of the Russian tradition along with one of the compilers of the
Philokalia.
EDIT: Since the question of ecclesial association has arisen, and since there are Latins here speaking on behalf of Easterners, I’d like to clarify that I’m Maronite.