i think all sins leads to death , if we do not repent...

Hebrews 6 tells us - it is impossible to renew them to repentance who leavs the principles of the doctrine of Christ.

Hebrews 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,

5 And have tasted the good word of God, and the powers of the world to come,

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.

There are different approaches through which Protestant theologians interpret this passage.

• The distinction of sin is of intensity. Venial and Mortal sins.

• The distinction of forgivable and unforgivable sins. Only blasphemy of Holy Spirit leads to death. Or apostasy alone leads to death.

• The distinction is of frequency of sinning.

• No distinction of sin, rather emphasis on wilfulness on sinning.

Charles Ellicott commentary states:

St. John only means that though prayer can do much for an erring brother, there is a wilfulness against which it would be powerless: for even prayer is not stronger than freewill. (Comp. 1Jn. 2:1; Luk. 22:31-32; Joh. 17:9; Heb. 7:25.)

There is a sin unto death.—The limit of intercession is now given: such conscious and determined sin as shows a loss of all hold on Christ. Such a state would be a sign of spiritual death. Hardened obstinacy would be invincible; and as it would not be according to the will of God that prayers, by the nature of the case in vain, should be offered to Him, St. John thinks that intercession ought to stop here. At the same time, he is careful not categorically to forbid it; he only says that in such cases he does not recommend intercessory prayer. (Comp. Mat. 12:31-32; Mar. 3:29; Heb. 6:4; Heb. 6:6; Heb. 10:26-27.) “His brother” is here, of course, a nominal Christian.

(17) All unrighteousness is sin.—Here St. John reminds them that all Christians might, at one time or another, stand in need of intercessory prayer, even those who, on the whole, might be considered as “sinning not” (because their permanent will was against sin, and for holiness), because every declension from the perfect righteousness of God is error or sin. Nothing that was not hopelessly deliberate need be considered a sign of absolute spiritual death. (Comp. 1Jn. 3:4.)

This is likely the unforgivable sin.

Matthew 12:32 (NIV)

32 Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.

(emphasis added here, too)

You can read more about this in the question Are there unforgivable sins?.

What is “sin that leads to death” in 1 John 5:16?

Earlier in John's letter he states that those who have no love for their fellow Christians "abide in death".

1 John 3:14 We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.

Even just a few verses earlier (5:12) John says that without Jesus there is no life.

1 John 5:12 He that hath the Son hath life; and he that hath not the Son of God hath not life.

It would seem that John is saying that the sin unto death is the rejection of or even disinterest in Jesus such that new and eternal life is not acquired. This state can be measured by a lack of love for other Christians.

Protestants actually provide a variety of interpretations of this passage. The "easy" answer, that this refers to the "unforgivable sin," is held by some, but others argue that the case for that interpretation is weak, and suggest several alternatives.

A couple of notes to begin:

• Protestant definitions of the unforgivable sin vary significantly, and these variations impact how this passage is interpreted.
• Many commentators point out that John is not necessarily forbidding prayer for sins that lead to death: he may be simply not commanding it.

Physical death

Adam Clarke references two possibilities in particular that he finds preferable, and both involve interpreting "death" as physical death, not spiritual.

The first, described by John Hewlett and held by John Rosenmuller and Henry More, is that the "sin unto death" is a crime deserving capital punishment. Thus, the government, and not God, should be petitioned in such cases.

The second possibility of this type was advanced by John Wesley. Citing the example of the disobedient prophet in 1 Kings 13, Clarke summarizes:

The sin unto death means a case of transgression, particularly of grievous backsliding from the life and power of godliness, which God determines to punish with temporal death, while at the same time he extends mercy to the penitent soul.

Spiritual death

The theologians described here broadly agree that the "death" mentioned here is spiritual death, and that it applies to those who demonstrate themselves, by their actions, to be outside the Body of Christ. However, they disagree on the definition of the "unforgivable sin." The first group thus holds that the two are the same, but the latter does not see a reference to it.

Reference to the unforgivable sin

The New Bible Commentary is straightforward on this point – John is referring to the unforgivable sin, which the authors see as a "state of sin, of being in rebellion against God":

Our Lord warns that he who blasphemes against the Spirit 'will not be forgiven' (Lk. 12:10) and it is this kind of thing that is in mind here.

John Calvin also sees the unforgivable sin as the referent: not a mere "partial fall" but "apostasy, by which men wholly alienate themselves from God." This corresponds to his view that the unforgivable sin can only be committed by the unsaved: he sees this passage as referring the "reprobate" and those "given up to destruction."

Matthew Henry is a bit more cautious, but still believes "blasphemy against the Holy Ghost" and "total apostasy [...] to be the sins chiefly intended by the apostle."

Not a reference to the unforgivable sin

Colin J. Kruse isn't convinced that John is referring to blasphemy against the Holy Spirit here, "because it fails to note the explanation given by Mark concerning the nature of this sin, that is, that it involves ascribing the miracles of Jesus to the work of the devil. There is no hint of this in 1 John."

Brooke Westcott, argues that the phrase "sin unto death" was borrowed from Rabbinic writings (cf. Numbers 18:22). Thus, "a 'sin unto death' would be a sin requiring the punishment of natural death. [...] Death in such a case was final exclusion from the Divine Society." Extending this to Christian society, then, he sees this sin as referring to any sin that by "its very nature excludes from fellowship," such as hatred of believers or denial of Christ: acts that reveal that one is not a part of the Body.

Kruse argues that from the context of the book, John is "very likely" referring to the sin of a group called secessionists, people who denied the incarnation and atoning death of Christ. Kruse sees this group as one of the "central issues being addressed by the letter," and thus finds it preferable to interpret the sin that leads to death to be "that of the unbeliever," particularly, these "antichrist" heretics.

Robert W. Yarbrough similarly sees the sin as those actions proceeding out of "a heart unchanged by God's love in Christ," and IVP New Testament commentary agrees:

The distinction between kinds of sin is not, therefore, a ranking of the seriousness of sins that believers commit. Instead, we have here an implicit distinction between kinds of sinners and sinning. "Sinning not unto death" is, paradoxically, sin in the realm of life, committed by one who has eternal life. [...] But where there is no confession, there is "sinning unto death," sin committed in the realm of death, sin that comes from and leads to death for the one who is guilty of it.

Summary

The Protestant interpretation of this passage is by no means clear cut, as evidenced by the number of variations advanced by these theologians. However, it's important to note that despite the wide-ranging rationales, ultimately two views emerge: (1) that this sin is naturally punished with physical death, and (2), that the sin is committed only by those outside the Body of Christ.

Thus, we find general agreement that John is not describing a sin that, if committed, sends a Christian to hell. Nor is he concealing some mysterious infraction that, if accidentally committed, results in spiritual death. If indeed he does have spiritual death in view, he refers only to sins that in themselves demonstrate alienation from Christ's church.

References:

• Calvin, Commentary
• Clarke, Commentary
• Henry, Commentary
• Hewlett, Commentaries and Annotations on the Holy Scriptures, V, 171
• IVP New Testament Commentary
• Kruse, The Letters of John, 2000, 192–94
• New Bible Commentary, 1970, 1270
• Westcott, The Epistles of St. John, 209ff.. This text also includes a number of quotations from church fathers on the subject.
• Yarbrough, 1–3 John, 2008, 309–11