Would you say this is a decent statement by Wm. Barclay of what John calls the “sin unto death” in 1Jn 5.16-17?

It is a fact of experience that there are two kinds of sinners. On the one hand, there is the man who may be said to sin against his will; he sins because he is swept away by passion or desire, which at the moment is too strong for him; his sin is not so much a matter of choice as of a compulsion which he is not able to resist. On the other hand, there is the man who sins deliberately, of set purpose taking his own way, although well aware that it is wrong.

Now these two men began by being the same man. It is the experience of every man that the first time that he does a wrong thing, he does it with shrinking and with fear; and, after he has done it, he feels grief and remorse and regret. But, if he allows himself again and again to flirt with temptation and to fall, on each occasion the sin becomes easier; and, if he thinks he escapes the consequences, on each occasion the self-disgust and the remorse and the regret become less and less; and in the end he reaches a state when he can sin without a tremor. It is precisely that which is the sin which is leading to death. So long as a man in his heart of hearts hates sin and hates himself for sinning, so long as he knows that he is sinning, he is never beyond repentance and, therefore, never beyond forgiveness; but once he begins to revel in sin and to make it the deliberate policy of his life, he is on the way to death, for he is on the way to a state where the idea of repentance will not, and cannot, enter his head.

The mortal sin is the state of the man who has listened to sin and refused to listen to God so often, that he loves his sin and regards it as the most profitable thing in the world.