This questions is somewhat complicated and requires a detailed answer.

First, we need to realize that the background of the letter of I John was that is was written – evidently – in response to a false teaching of the first century know today as Gnosticism.  Gnosticism took many doctrines and beliefs and there was, evidently, no one single consistent teaching concerning this false doctrine EXCEPT the belief that Jesus DID NOT come in the flesh.  Gnostics believed that flesh – as a material substance as opposed to spiritual – was evil so anything of flesh was evil thus Jesus could not possibly have come in the flesh.

Notice what John wrote in I John 2:18 to 22:

“Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have arisen; from this we know that it is the last hour. They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, in order that it might be shown that they all are not of us. But you have an anointing from the Holy One, and you all know. I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son.”

This passage defines without a doubt that the antichrist is the one who denies the Father and the Son.”

Next look at I John 4:1 to 6:

“Beloved, do not believe every spirit, but test the spirits to see whether they are from God; because many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; and every spirit that does not confess Jesus is not from God; and this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world. You are from God, little children, and have overcome them; because greater is He who is in you than he who is in the world. They are from the world; therefore they speak as from the world, and the world listens to them. We are from God; he who knows God listens to us; he who is not from God does not listen to us. By this we know the spirit of truth and the spirit of error.”

This passage is dealing with false teachers or false spirits, that is teachers or spirits that are false prophets gone our into the world – verse 1.

We are to test “every spirit,” verse 1, to see if it is a false prophet, and the test is, does the spirit confess “that Jesus Christ has come in the flesh.”  If it does, then we know it is from God.

HOWEVER if it does not confess “that Jesus Christ has come in the flesh” (to continue the context of verses 1 and 2) that spirit “is not from God” verse 3 and additionally, that spirit is the spirit of the antichrist, verse 3.

Then in verse 4 to 6 John explains that WE – Christians – are from God: we have overcome “them,” that is false prophets who deny Jesus: and we have overcome because He who is in you – Jesus – is greater that he who is in the world – the false spirit who denies Jesus, ultimately, the devil.

In I John 4:7 to 21, John explains God’s love, what it is, what it means, and the results of God’s love, culminating in the command in verse 21, “that the one who loves God should love his brother also.”

John continues in chapter 5 of I John by explaining that “Whoever believes that Jesus is the Christ is born of God; and whoever loves the Father loves the child born of Him.”  Then John identifies those who believe in verses 2 to 5 concluding with the statement, “ And who is the one who overcomes the world, but he who believes that Jesus is the Son of God?”  Showing the continuing context of I John concerning true prophets (spirits) as opposed to false prophets (spirits).

In verses 6 to 12 of I John 5, John is verifying that Jesus did in fact come in the flesh and that coming: it is not just a claim, it is a fact verified by “the Spirit who bears witness,” verse 7.  Then in verse 8 John adds two more witnesses making three total, the Spirit, the water, and the blood.  God’s witness is greater than the witness of men, verse 9, and He – God – “has borne witness concerning His Son,” verse 9.

Then John concludes this evidence in verses 10 to 12, “The one who believes in the Son of God has the witness in himself; the one who does not believe God has made Him a liar, because he has not believed in the witness that God has borne concerning His Son.  And the witness is this, that God has given us eternal life, and this life is in His Son.  He who has the Son has the life; he who does not have the Son of God does not have the life.”

We then get to I John 5:13 to 21 where John is closing his letter and in verse 17 writes, “ All unrighteousness is sin, and there is a sin not leading to death.”

Prior to that, however, John explains WHY he has written this entire letter in verse 13, “ These things I have written to you who believe in the name of the Son of God, in order that you may know that you have eternal life.”

In verse 14 and 15 John explains our relationship to God and how that relationship provides us our confidence when we pray.  Then in verse 16 we read, “If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this.”  The “sin not leading to death” appears to be ANY sin EXCEPT the one “leading to death” so, the question becomes, “What is the sin leading to death?”

To me, keeping the context of I John the sin leading to death is denying that Jesus came in the flesh and gave us eternal life AND that that coming and giving was from God Himself.  Whoever denies Jesus has NO hope of eternal life, thus that sin – denying Jesus – leads to death, that is eternal death.

This, then, is basically denying God and anyone who does that has no hope and has, therefore, committed a sin leading to death.

This also, to me, explains and defines what Jesus said in Matthew 12 where after Jesus cast a demon out of a blind and dumb man and then healed him so that the could see and speak the Pharisees accused Jesus of using the power of Beelzebul, the power of the demons, to heal the man and cast out the demon.  In response to the Pharisees Jesus said:

“Any kingdom divided against itself is laid waste; and any city or house divided against itself shall not stand And if Satan casts out Satan, he is divided against himself; how then shall his kingdom stand?  And if I by Beelzebul cast out demons, by whom do your sons cast them out? Consequently they shall be your judges.  But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you.  Or how can anyone enter the strong man's house and carry off his property, unless he first binds the strong man? And then he will plunder his house.  He who is not with Me is against Me; and he who does not gather with Me scatters.  Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven.  And whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him, either in this age, or in the age to come.”

ANYONE who could – as the Pharisees did – see demons cast out which indicated that they were subject, in some manner, to the power of the one speaking and then deny that power or attribute that power to Beelzebul was never going to recognize the power or God and thus was never going to submit themselves to God.  For people with that attitude and with that belief, there is not forgiveness because their heart is hardened to the point that they will never submit, the will never believe in God.

That it the same basic message John is explaining in I John 5:16 & 17 those who deny God, His power, His salvation, His Son, His plan, His gospel, are hardened to the point they are committing a sin that can never be forgiven because they will never recognize God for who He really is.