To those who do not believe in mortal and non-mortal (venial) sins, how do you understand this passage from 1 Jn 5:16-17:
 If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that.
 All wrongdoing is sin, but there is sin which is not mortal.
Is this not a clear explanation that not every sin is mortal, that although all sin is wrongdoing, some sins are not mortal? If so, why would one persist in not accepting the Catholic/Orthodox view that there are differing degrees of sins, mortal and venial. I’d really like to hear the interpretation of this passage by a non-Catholic.
The NIV translation of that verse (similar to the NKJV and KJV) is as follows:
If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that.
All wrongdoing is sin, and there is sin that does not lead to death. ***
Since elsewhere in scripture we are told that all sins are forgivable accept blasphemy of the Holy Spirit - that is what I have believed this verse to be referring to. I.e., it is Blashemy of the Holy Spirt which leads to death - so can’t pray about that, but can pray about other sins. To be honest - I never really gave much tought to the last part of the verse “. . . pray and God will give him life”.
See Matt 12:31 And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven
Now having said that, and upon reflection based upon your post (and I haven’t reflected in a while on this verse and those you cited), I can see a broader reading that these verses imply a need to pray about other sins - which otherwise could lead to death.
The implications seems to include not only the existence of sins that are non-mortal, but also that our prayers for the forgiveness or remission of non-mortal sins of another persons are effective for that other person. In other words, the passage seems to be teaching that, as members of the Body of Christ, our prayers for the healing of other members of the Body are efficacious. At least, that seems to be what the passage is saying. Those who have sinned, but have not cut themselves off by “mortal” sin, are able to obtain the spiritual benefits of the prayers of the other members of the body, for their own benefit.
I guess the physical analogy is that a hand that receives a burn (i.e. venial sin) can be healed by the action of the whole body because it is still attached, although injured. But where a hand is “cut off” from the body (i.e. mortal sin) it cannot receive the beneficial actions of the rest of the body.