The Danger of Unbelief

Hebrews 3:16-19

The author asks a series of questions about the historical incident that prevented the Jewish adults from entering the promised rest. Basically he is making the point that it is the very people that Moses led out of captivity that have been denied entrance into God's rest. They had seen the redemptive work of God in the Exodus but still their hearts tended towards unbelief. The author is stressing that unbelief leads to disobedience. The logical flow, according to the author is: unbelief leads to disobedience leads to discipline. In this case, the discipline is death and an inability to enter God's promised rest.
All of this relates back to the quote the author uses: "if you hear his voice, do not harden your heart." The warning is that a hardened heart leads to disobedience and disobedience leads to punishment. It is an exhortation to continue in obedience to the voice of God and to make sure that my heart is soft toward him and his direction. The author is hinting that there is a promised rest for all of those who continue to obey God. The Sabbath and the Promised Land are mere shadows that point to the final Sabbath rest available to those who obey.