I agree with much of what Jon Ericson has said but I think we can get even closer to the meaning of the "sin that leads to death" from the context of 1st John.
John is dealing with a division that has occurred in his church (1 John 2:18-19). Some have left, denying that Jesus' had a physical body (1 John 4:1-3). The young men of the congregation (2:12-14) are zealous for righteousness and want no part with the world. They possibly see the older men of the congregation (i.e. the fathers, 2:12-14) as capitulating with the world. John emphatically agrees with the young men; no true Christian ever sins.
"No one who continues to sin has either seen him or known him. Dear
children, do not let anyone lead you astray. He who does what is
right is righteous, just as he is righteous. He who does what is
sinful is of the devil, because the devil has been sinning from the
beginning. The reason the Son of God appeared was to destroy the
devil's work. No one who is born of God will continue to sin, because
God's seed remains in him; he cannot go on sinning, because he has
been born of God. This is how we know who the children of the devil
are: Anyone who does not do what is right is not a child of God..."
But then John turns his aim at those who claim to be without sin.
"...nor is anyone who does not love their brother and sister."
By love, John isn't talking about a feeling or an emotion. He's talking about a concrete action with a material effect.
16 This is how we know what love is: Jesus Christ laid down his life
for us. And we ought to lay down our lives for our brothers and
sisters. 17 If anyone has material possessions and sees a brother or
sister in need but has no pity on them, how can the love of God be in
that person? 18 Dear children, let us not love with words or speech
but with actions and in truth.
John stresses the fact that God loved tangibly in Jesus. The life that God gave in Jesus was able to be seen, handled and touched (1 John 1:1-2). The Anitchrists have denied that Jesus is the Christ (1 John 2:22, 5:1) which is to say they have denied that he is the Son of God (1 John 4:15, 5:5) which is to say they have denied that he came tangibly in the flesh (1 John 4:5). They believed he was water but not blood (1 John 5:6). And thus they practice what they believe, claiming to be spiritual without having to exhibit tangible, material love. And it is against this heretical belief and practice that John lashes out.
John tells his readers the reason why he wrote in 1 John 1:7-11.
7 Dear friends, I am not writing you a new command but an old one,
which you have had since the beginning. This old command is the
message you have heard. 8 Yet I am writing you a new command; its
truth is seen in him and in you, because the darkness is passing and
the true light is already shining.
9 Anyone who claims to be in the light but hates a brother or
sister is still in the darkness. 10 Anyone who loves their brother
and sister lives in the light, and there is nothing in them to make
them stumble. 11 But anyone who hates a brother or sister is in the
darkness and walks around in the darkness. They do not know where they
are going, because the darkness has blinded them.
The command they have had since the beginning is that they "love one another" (1 John 3:11). The new command is that anyone who does not tangibly love the children of God is still in darkness. There sins are not forgiven.
John states in 1 John 3:12-15,
Do not be like Cain, who belonged to the evil one and murdered his
brother. And why did he murder him? Because his own actions were evil
and his brother’s were righteous. 13 Do not be surprised, my brothers
and sisters, if the world hates you. 14 We know that we have passed
from death to life, because we love each other. Anyone who does not
love remains in death. 15 Anyone who hates a brother or sister is a
murderer, and you know that no murderer has eternal life residing in
The "sin that leads to death" is thus revealed to be a lack of tangible love for ones brothers and sisters. When we tangibly love those whom God loves the sin in our lives is muted.
18 Dear children, let us not love with words or speech but with actions and in truth.
19 This is how we know that we belong to the truth and how we set our
hearts at rest in his presence: 20 If our hearts condemn us, we know
that God is greater than our hearts, and he knows everything. 21 Dear
friends, if our hearts do not condemn us, we have confidence before
God 22 and receive from him anything we ask, because we keep his
commands and do what pleases him.
But when we lack tangible love it proves that we have "not passed from death to life." This is the sin that leads to death.