but where is the actual distinction made if you find it so clear?
In order for a sin to lead to death, it must meet three conditions-- a sin of grave matter,committed with full knowledge of the sinner, committed with deliberate consent of the sinner . This means that this sin cannot be done "accidentally." A person who commits a this sin is one who knows that their sin is wrong, but still deliberately commits the sin anyway. This means that they are "premeditated" by the sinner and thus are truly a rejection of Gods law and love.
In the Bible, St. Paul gives us a list of grave sins. He states that anyone who commits these sins shall not enter the kingdom of God. "Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God" (Galatians 5:19-20). Paul also tells the Corinthians, "know you no that the unjust shall not possess the kingdom of God? Do not err: neither fornicators, nor idolaters, nor adulterers, Nor the effeminate, nor liers with mankind, nor thieves, nor covetous, nor drunkards nor railers, nor extortioners shall possess the kingdom of God" (1 Corinthians 6:9-10). These sins constitute grave matter, and if they are committed willingly and with full consent, constitute sin unto death.