What does it mean to say Jesus died once for all?

In Hebrews it says Jesus died "once for all", meaning all sin was covered by His blood. Does this include unbelievers' sins? And if as a believer my sins were atoned for on the cross, why must I believe to be saved?

When the Bible declares that Jesus died once for all, it is discussing something very technical and specific:

Heb. 7:26 For it was fitting for us to have such a high priest, holy, innocent, undefiled, separated from sinners and exalted above the heavens;
Heb. 7:27 who does not need daily, like those high priests, to offer up sacrifices, first for His own sins and then for the sins of the people, because this He did once for all when He offered up Himself.

The writer of Hebrews is comparing Jesus’ sacrifice on the cross to the sacrifices offered by priests under the Mosaic law. The Mosaic sacrifices were repeated daily, because they were incapable of atoning for the sin of a human being. As the writer says:

Heb. 10:1 For the Law, since it has only a shadow of the good things to come and not the very form of things, can never, by the same sacrifices which they offer continually year by year, make perfect those who draw near.
Heb. 10:2 Otherwise, would they not have ceased to be offered, because the worshipers, having once been cleansed, would no longer have had consciousness of sins?
Heb. 10:3 But in those sacrifices there is a reminder of sins year by year.
Heb. 10:4 For it is impossible for the blood of bulls and goats to take away sins.

The very fact that these sacrifices were repeated was proof that they were not capable of removing sin. They had to be repeated because sin remained, and therefore the writer says they were merely a “reminder of sins.”

By contrast, Jesus’ sacrifice was “once for all.” His sacrifice will never be repeated because His death was sufficient to cover all sin. So the writer’s point is to compare the effectiveness of Christ's sacrifice under the New Covenant with the ineffectiveness of the sacrifices of the Old Covenant.

However, when the writer says “once for all”, he is NOT teaching on the application of Christ’s sacrifice. Christ’s sacrifice has the power to save all, but it will not be applied to all, since obviously many people will experience the second death (see Revelation 20). Later the writer of Hebrews says this:

Heb. 10:14 For by one offering He has perfected for all time those who are sanctified.

Notice the writer is now speaking about the application of Christ’s sacrifice, and here he says that Christ's sacrifice is applied to “those who are sanctified.” In other words, God’s elect are those being perfected by Christ’s sacrifice. Though His death is capable of saving all, it is applied only to the elect.

Regarding the necessity of faith and the consequence for unbelief, Pastor Armstrong covers these issues in detail in his Ephesians Bible study. We strongly recommend you listen to this study from the beginning.