First John 5:16 has been very confusing to many Christians... here is a simple analysis... read with open heart... u will catch something...

"If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them
that sin not unto death. There is a sin unto death : I do not say that he shall pray for it." 1John 5:16...

First of all....note dat dre is a difference between between Forgiveness and Repentance... to repent is to change entirely... while forgiveness is for born again, those who have changed....repentance is for those who need to b born again.... dre4, for u to enjoy forgiveness, u must have repented... and what makes u to repent is Salvation shown to u...

Back to 1john 5:16... the sin unto death is dos who refused to accept Jesus Christ as Lord... they deliberately enjoy sin and want it like dat... for this people... John is saying don't pray for forgiveness for them... it is not for them...(de cannot be forgiven because they will repeat sinning ).... what they need is prayer of salvation and repentance only...
when they now become Christians... de will make some mistakes which is sin not leading to death because God will forgive them as forgiveness is d children heritage... they must only confess d sin...(with intention not to continue in dat sin,born again dont enjoy repeating same sin ).... fellow chriatians shd dre4 pray for God's forgiveness for their brother who sin .... and pray for salvation for dos who have rejected Jesus by their lifestyle...

In summary... u sin unto death if u have not accepted Christ.... if u have accepted Christ and u born again... u dont sin unto death...

u make mistakes (sin )....such mistakes is not unto death... cos u have the Master... only ask for forgiveness with intention not to repeat it...

NOTE: Sin unto death is sin leading to hell directly.... sin not unto death is not leading to hell unless d person did not ask for mercy...