1 John 5:15-17 is making reference to any person *remaining in unbelief (sin) to the gospel of Jesus Christ Crucified till death* as *sin unto death* and no prayer should be offered for such because he/she is gone to hell-fire forever while any person still alive biologically and lives in unbelief (sin) can be prayed for and given the gospel adequately because there is possibility for *repentance and convertion for such a person* hence he declared *all sin is unrighteousness/wickedness before God* because all types/kinds of sin by persons in this 2-scenarios are *evil* in the eyes of God but *repentance* is feasible for those still alive biologically.
Great Grace Brethren!!!