Why Not Pray About Sin Leading To Death?

"If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that."             (1 John 5:16)

"If we receive the witness of men, the witness of God is greater; FOR THIS IS THE WITNESS OF GOD WHICH HE HAS TESTIFIED OF HIS SON. He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son. And this is the testimony: that God has given us (John, Peter, Paul, James etc.), and this life is in His Son. He who has the Son has life; he who does not have the Son of God does not have life. These things I have written to you who believe on the name of the Son of God, that you may know that you have eternal life,  AND THAT YOU MAY CONTINUE TO BELIEVE IN THE NAME OF THE SON OF GOD. Now this is the confidence that we (J,P,P,J etc.) have in Him, that if we ask anything according to His will, He hears us. And if we know that He hears us, whatever we ask, we know that we have the petitions that we have asked of Him."  (1 John 5:9-15) 

"We know that whoever is born (made by) God does not sin; but he who has been born of God keeps himself, and the wicked one does not touch him. We (J,P,P,J etc.) know that we are of God, and the whole world lies under the sway of the wicked one. And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. Little children, keep yourselves from idols. Amen." (1 John 5:18-21)

"But may the God of all grace, who called us  (J,P,P,J etc.) to His eternal glory by Christ Jesus, after you have suffered a while, perfect, establish, strengthen, and settle YOU. To Him be the glory and the dominion forever and ever. Amen. By Silvanus, our faithful brother as I consider him, I have written to you briefly, exhorting and testifying that THIS IS THE TRUE GRACE OF GOD IN WHICH YOU STAND. (1 Peter 5:10-13)

So, why not pray for a brother whose sin is leading to death? Because that one has transgressed "the true grace of God" in which believers must stand. They are failing to turn toward His commandments. These must understand why there is a vacancy of "the effective, fervent prayer of a righteous man (which) avails much"(James 5:16). Here, "judgement begin (s) at the house of God". When the righteous one is scarcely saved, where will the ungodly and the sinner appear?"(1 Peter 4:17,18)

You "make Him a liar" to the ungodly and the sinner when you WON'T keep His grace, His commandments, His words!