Sin unto death

1 John 5:16 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.

What is the sin unto death? Is it not rejecting Jesus as Lord and going out into the world? Where is the proof? I think you will know through the Spirit's witness if someone is not of God. Note, that John is speaking of a brother, a professing believer. A professing believer may not be truly committed to Christ.

I have seen people who left being a Christian who were enemies of the cross. They became antichrist like. Wolves doing all they could to destroy the faith of other people. We see them in the world going to court to stop the display of the ten commandments or placing anti God billboards in public places.

1 John 2:18 Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
1 John 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us. 
1 John 2:20 But ye have an unction from the Holy One, and ye know all things.

Heb 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Heb 6:5 And have tasted the good word of God, and the powers of the world to come,
Heb 6:6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

I was one who for a time fell away, don't be too swift to abandon someone who has fallen away. It may be that a great trial or sorrow has come upon the person. If this is the case we should go to them seeking their restoration.

Gal 6:1 Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted.
Gal 6:2 Bear ye one another's burdens, and so fulfill the law of Christ.

James 5:19 Brethren, if any of you do err from the truth, and one convert him;
James 5:20 Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins.

Some are to be sought after and some are to be avoided.