Christ's sacrifice applies only once for each person, and if we reject God's grace, it cannot be applied again. This is why willing apostasy is so terrible and why the apostles fought so strongly against heresy in the first century. The eternal lives of thousands of God's people were at stake!
In a more passive way, sin can lead to eternal death by continued neglect. The sinner may know he should repent of sin, but because of lethargy he never bothers to overcome it. He is apathetic; he just does not care. The Laodicean attitude (Revelation 3:15-19) comes dangerously close to this type of sin, and if not repented of, it can lead to the unpardonable sin.
