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:
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.
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.
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.
- 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