The Apostle John talks about the sin that leads unto death, so what is it and can we commit it?
The Wages of Sin
Before we answer “What is the sin that leads unto death” (1 John 5:16), we should recognize that all sin leads to death. So, what is sin? The Apostle John says that “sin is lawlessness” (1 John 3:4b), or breaking God’s Law, which we’ve all done. Before we were saved, all we were doing was storing up our wages…the wages of our sins…and the wages of sin is death, or eternal separation from the presence of God (Rom 6:23a). Those who’ve been brought to repentance and faith in Christ have been given the free gift of eternal life (Rom 6:23b). It is free to us…but costly to Christ, but Jesus paid for the wages or earnings that would have been due on the Day of Judgment. That meaning, what we deserved was placed upon Him Who didn’t deserve it. That’s amazing grace. Even though we were due the wages of death, we put our trust in Christ and have been spared the wrath of God. 
If sin is not repented of, it will accumulate and do the person spiritual harm and damage their fellowship with the Father. Their relationship might not change; He is still their Father, but their fellowship has been damaged due to unrepentant sin. We can confess our sins and God will cleanse us from all our unrighteousness (1 John 1:9), and we need that since we all sin (Rom 3:23; 1 John 1:8, 20), but when a person falls into a serious and besetting (ongoing) sin and doesn’t bother to confess that sin, it could become a sin that brings us home to heaven. The command is still, “You be holy, for I am holy” (1 Pet 1:16). It doesn’t say “You be perfect,” because that’s not humanly possible. All Christians still fall into sin and will never be sinless in this world, but over time we should be sinning less, so we fall into sin. The sinner doesn’t just fall into sin; they dive into it and enjoy the swim. We should be sinning less over time. That’s called “sanctification.” We are supposed to be growing in holiness and the knowledge of the Lord. Sometimes God will discipline us as “The Lord disciplines the one he loves, and he chastens everyone he accepts as his son” (Heb 12:6). If discipline fails over time, the besetting or ongoing sin of a serious nature may be the sin that leads to their “premature” death. Maybe that’s what the Apostle John meant in 1 John 5:16.
The Sin unto Death
I do not believe God will bring home someone just because they fall into habitual sin, however God reserves the right to bring us home at any time He wants, however if the habitual sin is for a married man or woman to be committing adultery and they show no signs of repentance, that kind of a serious sin might be the sin that leads unto death (1 John 5:16). A sin like that of adultery can devastate families and relationships, and where the adultery is ongoing and not repented of, then there will be many innocent victims. Whoever commits adultery sins against their own body (1 Cor 6:18), meaning it can have physical and spiritual consequences. Every other sin seems to be committed outside of the body. The Apostle Paul says, “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body” (1 Cor 6:18). When the adulterating parties are “joined to a prostitute [they become] one body with her” or him, since “The two will become one flesh” (1 Cor 5: 16). We are to be joined only with Christ. The one who proclaims himself or herself to be a Christian, and yet continues to live in sexual immorality, is deceiving themselves since “those who practices such things will not inherit the kingdom of God” (1 Cor 6:10; Gal 5:21). Every believer must “Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous” (Heb 13:4), and that judgment might even mean the sin that leads unto death.
In my life, I’ve had this question asked numerous times, and I like what the late Dr. J. Vernon McGee said about the sin that leads unto death. He believed it is that one serious and besetting sin that has not been repented of or has not been overcome. Even though God says we can do all things through Christ, but only in His strength (Phil 4:13), some choose to hold onto a sin that they’ve been committing for years…perhaps their entire adult life. If that sin involves sexual immorality, it may be the sin that leads unto death. It might be different sins for different people, but it’s probably not a sin that we fall into and repent of and confess to God. It may be a sin that we don’t even ask forgiveness for anymore because we’re still committing it and have no plans to stop. If that is the case, then seek the prayers of the saints; from your church as “unspoken needs,” and seek counseling.
God has equipped many Christian counselors and pastors for establishing strategies for overcoming sexual addictions like pornography, alcohol or drug abuse, or an ongoing affair, either inside or outside of marriage. God has not left us helpless. First of all, we have Him to help us in our overcoming and He uses others as a means to overcome these terrible sins. Seek help today from the Lord and from His saints. They are eager to serve you, and you might possibly avoid that sin that leads unto death (1 John 5:16).
Here is some related reading for you: Bible Verses About Sin: 10 Important Scriptures 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.