This is a tough question for Protestants because of your rejextion of God-given Authority.
As the Bible admionishes us - there IS a hierarchy within the Church and there ARE those who are"over us" in Christ:
1 Thess. 5:12
We ask you, brothers, to respect those who are laboring among you and who are OVER YOU in the Lord and who admonish you
1 Tim. 5:17
Let the elders that RULE WELL be counted worthy of DOUBLE HONOUR, especially they who labour in the word and doctrine.
1 Cor. 12:28
Some people God has designated in the church to be, FIRST, apostles; SECOND, prophets; THIRD, teachers; then, mighty deeds; then, gifts of healing, assistance, administration, and varieties of tongues.
As for HOW we know which sis are Mortal and which are Venial - Christ left is with a CHURCH to teach us (Matt. 16:18-19, Matt. 18:15-18, Matt. 28:19, Luke 10:16, John 16:12-15). He did NOT leave us as orphans (John 14:18) in total chaos.
- Minor (venial) sin damages our relationship with God. The Bible calls this "sin that does NOT lead to death".
- Grave (mortal) sin severs that relationship. The Bible refers to it as “sin that leads to death”.
1 John 5:17
If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. All wrongdoing is sin, but there is sin that does not lead to death.