What does Hebrews 4:9 mean?

Hebrews chapter 3 explained that Israel's disobedience cost an entire generation their inheritance in Canaan. But it was only those who were disobedient who lost out on the Promised Land (Hebrews 3:16–19). In this chapter, the writer has been explaining that his Christian readers should not assume they have completely missed out on their "rest." This is not a question of salvation, but of eternal rewards. While some false teachers claimed that others were too late to receive their inheritance in Christ (2 Thessalonians 2:1–3; 2 Timothy 2:17–18), this is not the case.

Prior verses explained that the "rest" of Israel in Canaan was not the ultimate, permanent rest offered by Messiah (Hebrews 4:8). This is why writers long after Joshua, such as David, appealed to the people to seek after God's rest "today," as in Psalm 95. Here, the point is made that since God's seventh day Sabbath rest from creation (Hebrews 4:3–4) is ongoing, the "rest" He offers is available right now, to those willing to trust and obey.

As mentioned elsewhere in Hebrews, this is not about eternal salvation. Canaan, in this instance, is not a symbol of heaven. God will save for all eternity those who come to Him in faith (John 6:39–40), but He will only give an inheritance of rewards to those who "hold fast" to their faith (Revelation 2:26–27).

This rest is only obtained by following the examples of God Himself, and avoiding the example of Israel. God rested when His work was complete. Israel missed out on their rest when they failed to complete their works. So, as the next few verses will explain, saved Christians need to "strive" in order to complete the work God has given us.