What does Hebrews 9:27 mean?

This verse represents a key component of Christian theology. In contrast to religious systems which teach that man is constantly re-born, or vanishes into nothingness, the Bible is very clear that all men experience a single permanent death. Once this happens, each person is accountable for their actions in life. This verse also, indirectly, reminds the reader that the real judgment for sin comes after death. God can, and does, use earthly circumstances in order to prove His will. Sometimes, that means negative consequences for sin. For the most part, however, God's real work of judgment on sin happens after this life is over; at that point, each person is held accountable for their own life.

In the context of this passage, the writer is also making a point about the sacrificial death of Jesus Christ. As recently mentioned, the sacrifice offered by Jesus is not like that of the old covenant. Animal sacrifices had to be offered repeatedly, since they could not perfectly atone for sin. Christ was able to offer a single, "once for all" sacrifice which absolutely covered the payment required for humanity's salvation.

Between verses 27 and 28, the point is that Christ's first appearance, leading to His death, was for the purpose of judgment of sin. Sin was judged on the cross, by the sacrifice of Jesus Christ. When Christ comes again, in the future, His arrival will not be for the purpose of atoning for sin. Instead, it will be to apply what has already been accomplished, and to usher in the final judgment (Titus 2:13). Just as man's one death leads to judgment, so too does Christ's one death lead to judgment—but for men, rather than for Jesus.