We might want to take this to the Wesleyan forum. The
www.umc.org website is not responding right now, so I can't check how the UMC views this issue. However, I will say I am using the Wesleyan quadrilateral in my approach.
You are thinking of 5:16-17. I find those verses perplexing because in verse 16 John says there is
a sin that leads to death, but doesn't tell us which
one that is: "If you see a Christian brother or sister
[fn] sinning in a way that does not lead to death, you should pray, and God will give that person life.
But there is a sin that leads to death, and I am not saying you should pray for those who commit it.
Verse 17 says "All wicked actions are sin, but not every sin leads to death." That is not consistent with verse 16. Now John seems to be saying that there are multiple sins that can lead to death, not just one.
Other NT writers disagree with John. Paul says in Romans that all sin leads to death:
"Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned" 5:12 Later in verse 21 Paul says that we get out of death by grace: "That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord. "
Paul repeats this in 6.23 "For the wages of sin [is] death; but the gift of God [is] eternal life through Jesus Christ our Lord. " Paul doesn't say that some sin leads to death and others don't.
James agrees with Paul in 1:15 "Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. "
So John is the odd man out. I don't see anywhere that Jesus makes distinctions between sins. In fact, that is one of the points of John 8: the men had no right to stone the adulteress because they had all committed their own sins. What's more, Jesus makes a point that even
desiring a woman in your thoughts is equal to adultery. Even being angry with someone is equal to murder. That message is equalizing sins. Since John never specifies which sins are which, I tend to think that Paul and James are more correct.